For the fish fillet gratin, dry and acidify the fish fillet, add salt and pepper.
Sauté the mixed chopped vegetables in a pan in a little butter and sauté briefly. Pour half of the vegetables into a baking dish, place the fish fillet on top and cover with the remaining vegetables.
Mix wine with whipped cream, add some lemon juice and spread over the fish vegetables. Sprinkle with breadcrumbs and cheese and spread small flakes of butter on the surface.
Bake the fish fillet gratin for 15 to 20 minutes in the preheated oven at 220° C and serve.