When Saulo Jennings heard he would be cooking for an heir to the British throne, the Brazilian chef knew he wanted to showcase the star ingredient of the Amazon: a huge, fleshy river fish called pirarucu.

That heir, Prince William, was hosting an environmental awards ceremony by the Earthshot Prize for 700 guests in Rio de Janeiro in early November, his first visit to Brazil. Creating the canapés seemed like the perfect job for Mr. Jennings, an acclaimed chef from the Amazon known for using sustainable ingredients from the rainforest.

But there was a catch: The menu had to be 100 percent vegan, Mr. Jennings said he was informed by the Earthshot organization.

At first, convinced he had misunderstood, the chef said he offered to add a vegan option to the menu. Then, he said, when he was told he could not use any of the cherished river fish species central to his dishes and to the Amazon region’s cuisine, he grew offended.

“It’s like asking Iron Maiden to play jazz,” Mr. Jennings said in an interview on Friday. “It was a lack of respect,” he added, “for local cuisine, for our culinary tradition.”

After much coaxing by the museum that will host the event, Mr. Jennings said he created a vegan menu inspired by Amazonian ingredients like cassava root, jambu leaf and Brazil nuts. But by then, negotiations over the catering of the event had broken down. And now, he is out as the caterer for the event, scheduled for Nov. 5.

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