Galician Tuna Empanada with Melting Onions

One of my favorite Galician empanadas features good canned tuna, tons of slowly cooked onions and bell peppers, and the pungent accent of green olives to balance the oniony sweetness. Tuna is the simplest option, but you can use this recipe as a blueprint for any Galician seafood empanada, using flaked, cooked, white-fleshed fish, sardines, or sliced raw scallops, for example.

In Galicia empanadas are often made with a wheat or cornmeal dough purchased from a bakery or with puff pas