Cut the onion and mushrooms and sauté them in a little oil under a lid. In the meantime, clean the carp and rub it with salt and ground black pepper. To the onion, add the finely chopped tomatoes, chopped walnuts, chopped parsley, salt, pepper and wine.
Wait until the liquid evaporates a bit and remove it from the heat. Fill the carp with it, sew it up and transfer to a greased baking tray. Cover it with round tomato slices and add the remaining melted butter.
In the tray, pour in 1 cup hot water and put it in to bake.