Tuna with Sweet-and-Sour Onions

Here’s a perfect preparation for tuna in summer—the time of year when the fish is plump and unctuous. The recipe comes from Trapani in Sicily, where tuna fishing has been a major industry for centuries. Thin tuna steaks are fried, drenched in a sweet-and sour-onion sauce, and refrigerated overnight or for several days, which allows the flavors to meld.