Winter flounder—a.k.a. lemon sole when it’s big, and blackback flounder when it’s not—takes its name from the fact that it swims inshore to warmer waters during the winter months.

Reduce the heat and simmer the fish for 10 minutes, or until just cooked. Meanwhile, to make the warm tomato vinaigrette, warm the oil, vinegar and red onion in a small frying pan. After a few ...