Wash the plaice fillets, dab dry, sprinkle with lemon juice and season with salt. Leave to stand. Peel onion and garlic. Chop the onion finely, press the garlic through a garlic press.
Clean and wash the zucchini, cut them in half lengthwise and cut into thick slices. Heat oil in a pan and braise the zucchini in it. Add onions and garlic and fry. Pour on 200 ml water, stir in Zucchini-Fix and boil up again briefly.
Season to taste with salt and pepper. Put the plaice fillets in a greased baking dish and spread the courgette vegetables over them. Mix parmesan and breadcrumbs and sprinkle over the casserole. Spread the fat in flakes on top.
Bake in the preheated oven (electric oven: 200°C/ gas: level 3) for 20-30 minutes. Serve sprinkled with fresh thyme as desired. Serve with rice or fresh baguette.