Peel, wash and cut the potatoes into pieces. Cover and cook in salted water for about 15 minutes. In the meantime peel and finely dice the onions. Peel, wash and chop the carrots. Rinse and drain the prawns. Rinse fish, dab dry and cut into approx. 3 cm cubes.
Add the frozen peas to the potatoes, bring to the boil again and continue cooking for about 5 minutes. Heat 2 tablespoons of butter in a saucepan. Brown the onions and carrots in it. Dust with flour and sauté briefly. Add 1⁄4 l water and milk while stirring. Stir in stock, bring everything to the boil. Add laurel. Simmer at low heat while stirring for about 5 minutes. Season with salt and pepper.
Preheat oven (electric cooker: 200 °C/circulating air: 180 °C/gas: see manufacturer). Drain potatoes and peas. Add 3 tablespoons butter, 200 ml milk and lemon peel. Mash everything finely. Season to taste with salt and nutmeg.
Add the prawns, fish and lemon juice to the sauce and bring to the boil once. Remove from the heat and spread in a large casserole dish (approx. 4 l capacity). Carefully spread the potato mixture on top. Grate the cheese finely and sprinkle over it. Bake in a hot oven for 25-30 minutes.