British supermarkets are brimming with packages of coronation cupcakes, hoppy coronation ale and coronation crisps seasoned to taste like king prawn cocktail. There are even Jack Russell-themed cakes in honor of the king and queen consort’s rescue dogs, Bluebell and Beth.

The royal couple likely won’t eat any of it. King Charles III and Queen Camilla lean heavily on fruit and vegetables, preferably organic and from their own gardens. She loves fish and salads. For breakfast, he requires a selection of six honeys and a special mix of muesli, and to end a meal, a plate of local cheese. To lower his carbon footprint, the king goes vegetarian two days a week, and gives up dairy on another.

But can Britain’s first green-minded monarch persuade a nation that loves Sunday roast, chips and chicken tikka masala to embrace the locally grown, organic, climate-friendly diet that he follows?

There is undoubtedly power in how a nation’s leaders and their families eat. Jacqueline Kennedy installed a young French chef from New York in the White House kitchen, ushering in a new frontier of sophisticated dinner-party diplomacy. The Obamas planted a vegetable garden and used it to to promote local food and healthier school lunches. President Trump served a fast food buffet to a championship football team to underscore his fight with Democrats over a government shutdown and to burnish his populist image with what he called “great American food.”

When it comes to making lasting change in a nation’s food culture, the king has an edge that American presidents don’t, said Jamie Oliver, the English chef and television personality who tackled unhealthy school lunches. He has worked with Charles on a number of projects, and shared a few meals and several “lovely cups of tea” with him.

“People in the public eye come and go, but not the crown,” Mr. Oliver said, “so he’s been very important for progress in the U.K. That consistency and relentlessness has extraordinary value, because his secret weapon isn’t just being king — it’s time.”

Charles was an early adopter of organic agricultural practices back when it was novel enough to get him a good drubbing from the press and even members of his own family. It didn’t help that he was an advocate for talking to plants and playing music to make milking a calmer experience for his cows. He even converted his blue 1970 Aston Martin, a gift on his 21st birthday, to run on bioethanol derived from surplus English white wine and whey left over from making cheese.

But if there is a royal last laugh to be had, he had it. Charles turned 1,100 acres next to Highgrove House, his estate west of London in Gloucestershire, into an organic farm that eventually supplied meat and produce for Duchy Originals, a company he founded in 1990. It has become a multimillion-pound food brand in a lucrative partnership with the upscale grocery chain Waitrose. Profits go to charitable causes, and it is likely that the king’s elder son, Prince William, will take over.

Small farmers in Britain see Charles as a champion, and his views on climate change and regenerative farming have made him a hero among some in the agricultural progressive class.

“When you shake hands with him you will understand what I mean when I say he has farmer’s hands,” said the chef and cookbook author Romy Gill, who lives about a 20-minute drive from Highgrove House.

Influencing farming practices is one thing. Convincing the dining public is another, as evidenced by the current outrage over a royal recipe for quiche. (More on that below.)

“There are, as you can imagine, some slightly different views on the royal family here,” said Calum Franklin, a British chef and author who rose to fame as a craftsman of meticulously researched and intricately designed traditional savory pies at the Holborn Dining Room in London.

Mr. Franklin is contributing his own dish to the coronation food madness. He has worked for nearly a year to develop a limited-edition Crown Jewel Celebration Pork Pie in collaboration with the pie maker Dickinson & Morris, which will make 500 of the intricately designed pastries by hand using British-bred pork and breast meat from pheasant, the king’s favorite game meat.

To lend it a bit of drama, Mr. Franklin fashioned the top crust after the sovereign’s orb, a golden jewel-encrusted globe and cross from 1661 that is presented to each new monarch as a reminder that their power derives from God.

Savory pies have played a pivotal role in British tradition. “Pies have been up and down in popularity based on royal banquets, and we can see that through history,” Mr. Franklin said. The term humble pie, for example, comes from pies made with umbles, or scraps of meat and offal that fed peasants who were seated far away from royalty at banquets.

Details about what the king and queen consort will eat to celebrate the coronation to remain sparse, though people who have cooked for Charles say menus will be simple and seasonal, most likely centering on his favorite protein, British lamb.

The table is also likely to hold cheeses like Stilton and Cornish Brie, honey from Highgrove and, as always, a little pot of the king’s custom blend of seasoning, which is heavy on the celery salt.

The royal family is encouraging Britons to cook for a series of street parties throughout coronation weekend. The main event, called the Coronation Big Lunch, will be held on May 7, a day after the new king is crowned.

Menus will include everyday food like sausage rolls, scones and finger sandwiches, but Buckingham Palace has sent out suggested recipes for home cooks, including roasted rack of lamb with an Asian-style marinade, hard-shell shrimp tacos with pineapple salsa, and grilled eggplant with a dressing of mango chutney, yogurt and curry powder.

But the real attention-getter has been the coronation quiche, the official dish

selected by the king and queen, and developed in the Buckingham Palace kitchen by the chef Mark Flanagan. (The last time a monarch was crowned, in 1953, the official dish was coronation chicken.)

The new dish, which was quickly nicknamed Quiche Le Reign, is built with spinach, tarragon, broad beans (called favas in the United States) and Cheddar, a nod to the cheesy eggs the king sometimes likes for breakfast. The crust is made with lard — that’s just one small part of the controversy — but the royal chefs advise that store-bought is fine.

As a culinary offering, the dish has drawn mixed reviews. Dame Prue Leith of “The Great British Bake Off” declared it “a really good quiche.”

Others pulled no punches. “It’s about as unimaginative as it gets,” said Mr. Franklin, the pie chef.

“A huge country full of unique cuisines and THIS is the crap you choose??” one Twitter user commented. “It’s 2023, not 1973. Are we having vol-au-vents and cheese with pineapple on a stick with it?”

The recipe’s reception quickly devolved into a political beef. The Reform Party called the dish “foreign muck,” and said pie would be a more patriotic choice. The Socialist Party asked whether the quiche would be available at food banks.

The recipe prompted some to chide Charles and Camilla for suggesting an egg dish in the wake of a national egg shortage brought on by avian flu.

Others simply pointed out that food prices were soaring. “Read the room, poshos,” one woman tweeted. “People can’t afford their weekly shop anymore.”

Perhaps the crown won’t have as much of an impact on British food culture as the green king might have hoped. At least not right away.

“Normally, I would say yes, there would be quite an influence,” said Darren McGrady, a British chef in Dallas who traveled with and cooked for Queen Elizabeth II, Princess Diana and other members of the royal family in the 1980s and ’90s.

“The only thing putting a damper on it right now is the economy,” he said. “A lot of people are hurting. When it comes to buying food to nourish and fill the belly or buying organic, they are going to choose filling bellies.”

Whether the king will remain a vocal advocate for progressive food and agricultural policies is up in the air. His mother maintained a strict political neutrality, and Charles knows he will have to temper his impulses to be outspoken.

“I’m not that stupid. I do realize it’s a separate exercise being sovereign,” he said in a BBC interview in 2018. “The idea that somehow I’m going to carry on exactly in the same way is complete nonsense.”

Still, people who have seen in him a champion for environmental causes believe he won’t go quietly into the royal night.

“He is in a completely unique position to change how so many people eat every day,” said Alice Waters, the California restaurateur and agricultural reformer, whom Charles once taught to weave a hedgerow. “If he didn’t talk about regenerative agriculture and climate, I would be shocked.”