Wash the potatoes and cook covered for about 20 minutes. Quench, peel and let cool
Peel the onion and cut into fine slices. Fry in hot oil until translucent. Add frozen spinach and almost 100 ml water. Steam covered for about 10 minutes. Season with salt and pepper. Stir from time to time
Crumble the toast. Wash and chop herbs. Mix with crumbs and parmesan. Wash fish, dab dry and cut into 4 pieces. Sprinkle with lemon juice, salt. Cut potatoes into slices and season.
Place the potatoes and spinach in 4 small or 1 large greased baking dish. Pour on the stock. Place fish on top. Spread herb mixture and 2 tablespoons of fat in flakes on top. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/gas: level 3) for about 20 minutes