Peel and cut the shallots and garlic. Brown them for 5 minutes in a frying pan with the butter.
Add the white wine, the citron juice from half of the citron, the chopped fresh parsley and crushed tomatoes.
Remove from the heat and add the breadcrumbs, salt, pepper and mix well.
Preheat the oven to 180 degrees Celsius.
In an oven form place the fish fillets. Then spread the mixture in a thin layer over the fish fillets. Sprinkle with some olive oil.
Bake for 15 minutes in the middle of the oven. Make sure the fish is not getting dry.
Take out the gratin and serv with vegetables and cashew nuts.