Sprinkle the fish with salt and pepper. Dust both sides with flour. In a large skillet, heat the oil. Fry the fish, turning once, for 10 to 20 minutes, depending on the thickness of the fillet, until it is crusty and brown.
Meanwhile, sauté the onion in the butter over medium heat until golden, about 10 minutes.
Transfer the fish to a shallow flameproof casserole. Strew the cook