Parece que o que você leu estava correto. De Fine Seafood Co aqui está uma descrição do peixe John Dory. (Como muitos nos EUA não serão familiares).
John Dory Often named St Peter’s fish (St Pierre in France, Janitore in Spain), as he is alleged to be the origin of the distinctive dark ‘thumbprint’ (or spot) on its side as a thank you for some help with his tax! The English name John Dory arrived from the French ‘jaune doré’ meaning ‘golden yellow’ – a good description for this unusual looking fish, which varies in size from smaller 230-450g fish up to 2kg. Because of the large head (like Monkfish, this accounts for half its weight), John Dory is best filleted, but beware the low yield (around 35%) and some sharp nasty spines, which require extra care when filleting. The flesh is creamy-white, with a dense texture similar to Dover Sole, which holds up well during cooking. Ideal pan-fried or grilled John Dory works well with Mediterranean flavours, salsas, and peppery sauces. It may be an expensive fish with a low yield – but it’s worth it!
(ênfase minha)
Por minha pesquisa, Dover Sole parece ser a primeira escolha a ser usada como substituto de John Dory. Outro que eu encontrei mencionado frequentemente era o Turbot. Mas isso parece ser ainda mais caro do que John Dory.
Outros listados como possíveis substitutos em Cookipedia são: Bass, Grouper, Snapper, Emperor e Halibut. / p>
Aqui está um link para o Guia da Fine Seafood Co com descrições de muitos outros peixes. (Nenhum endosso pretendido - apenas boa informação.)