1 Heat 2 tablespoons of the butter in a skillet, add the onions and saute for about 5 minutes. Preheat the oven to 375Â°F.
2 Using half of the remaining butter, grease an ovenproof dish. Place the tomatoes and fried onions in the bottom and then sprinkle with the sugar. Add salt, plenty of pepper and the vinegar. Arrange the fish fillets on top.
3 In a small saucepan, warm the rest of the butter and brush the fish with the melted butter.
4 Cover the dish with a lid or aluminum foil and bake in the preheated oven for 20 minutes. Serve sprinkled with the chopped parsley.