I'm looking for a short story by Isaac Asimov about someone who creates dishes not in the kitchen but by combining ingredients in his head.
Could it be you're thinking of "Good Taste" (readable here)? It's the closest match I can think of, except that it's kind of the reverse of what you're asking about. It involves a chef using real ingredients rather than the simulated forms from a computer.
Chawker Minor returns from his 'Grand Tour', including a visit to Earth, to his home on Gammer, one of several artificial satellites orbiting the Moon. The introverted society of Gammer specialises in artificial computer-designed food flavourings much in demand in Earth, to the point of shunning "natural" food grown in "dirt", and Chawker is inspired to enter the annual competition for flavouring, using something new and radical.
Despite the disapproval of his parents and elder brother, Chawker Minor does design a new flavouring which wins the competition. Asked by the Grand Master, who can taste and analyse flavourings to the smallest detail, to explain his successful and intriguing entry, he reveals that he has not used artificial computer-designed molecules, but an actual raw ingredient, garlic, maintaining that no assemblage of molecules may duplicate the complexity of a living organism.
The Grand Master, and all Gammer society, are revolted by this breach of good taste. Chawker Minor is disavowed by all and exiled from his home.
My search query was isaac asimov story about a chef