Clean and wash the fish, dry them. Peel the onions and cut them into round slices, the tomato and lemon into slices.
Lay out a tray with foil and pour in the oil, first distribute the onions, tomato and lemon slices along the bottom. Salt the fish to taste and lay them on top, sprinkle black pepper, finely chopped parsley and pieces of butter over the fish.
Cover with foil very well to make a tight packet. Bake in a preheated 428°F (220 °C) oven about 40 min., uncover the foil for the final 10 min.
Serve on a bed of potato salad or fries.