Clean the zucchini and leek and cut into slices or rings. Finely dice the onion and garlic.
Heat 2 tablespoons of the oil in a pan and brown the fillets all over. Season with salt, pepper and lemon juice and place in a baking dish. Heat the rest of the oil in the frying fat and sauté the onion and garlic in it. Add the remaining vegetables and fry briefly. Season with salt and pepper and pour everything over the meat.
Grate cheese (amount as desired). Mix the cream and mustard and season with a pinch of sugar, salt, pepper and paprika powder. Pour the sauce over the ingredients in the baking dish and sprinkle everything with cheese.
Bake in a preheated oven at 200 degrees for 20 to 25 minutes. Garnish with parsley before serving.