Mix the mustard and ketchup together, season the fish fillets with salt and pepper and spread the mustard and ketchup mixture on both sides. Steam the onion cubes in a little oil until soft. Place the fish fillets next to each other in a baking dish, one after the other pour the tomato slices, the steamed onion cubes and the pickled cucumber cubes over them. Finally, sprinkle everything with cheese. Bake at 180 ° for 40 minutes.