Clean the leek and cut into rings. Sauté in melted butter in a pan (add some water if necessary). Season with salt and pepper. Spread the leek in a baking dish.
Salt and pepper the fish fillets. Brush with mustard and place on the leek. Wash the tomatoes and cut them into cubes. Season with salt and pepper and spread on the fish. Stir the crème fraîche with a little milk until it becomes a little more liquid. Spread over the gratin (does not have to be completely covered).
Bake in the preheated oven at 200 ° C for approx. 25 - 30 minutes.