Grease a baking dish with herb butter. Brush the fish fillets with mustard and pepper. Place the fillets in the pan. Slice the tomatoes and mozzarella and spread them on the fish (with the cheese being the top layer). Salt and pepper.
Tip: If you like, you can put a crushed garlic clove on cheese and vegetables.
Put the dish in the preheated oven at 200 ° C top / bottom heat and cook the fish for 30 minutes. Sprinkle the dish with fresh basil before serving.