Wash the potatoes and pre-cook them in covered water for about 15 minutes. Then quench and let it cool down a bit.
Rinse fish fillets and pat dry. Sprinkle with lemon juice and season with salt and pepper. Cut dried to-mats into fine strips. Peel and finely dice onions and garlic. Wash thyme, shake dry and pluck off the leaves.
Spray a baking tray or a large casserole dish with a little oil or spread it thinly. Peel the potatoes, cut them into slices and spread them on top. Season with salt and pepper. Spray potatoes thinly with oil or sprinkle with a good 1 teaspoon of oil.
Place the fish fillets on top. Sprinkle with tomatoes, onions, garlic and thyme. Cook in the preheated oven (electric cooker: 200 °C/circulating air: 175 °C/gas: level 3) for 20-25 minutes.