Peel, wash and cut the potatoes into pieces. Cook in boiling salted water for about 15 minutes. Peel and chop the onions. Peel and chop carrots. Wash fish, dab dry and cut into large cubes. Wash the prawns and let them drip off.
Add peas to the potatoes and simmer covered for about 5 minutes. Melt 50 g butter in a saucepan, fry carrots and onions in it. Dust with flour and sauté while stirring. Gradually add 250 ml milk and stock. Add laurel, season with salt and pepper. Simmer at low heat for about 5 minutes. Stir again and again.
Drain the potatoes and peas and let them evaporate briefly. Add 50 g butter, 200 ml milk and lemon peel. Mash everything finely. Season to taste with salt and nutmeg. Add lemon juice, fish and prawns to the sauce, bring to the boil once. Remove from the heat and spread in an ovenproof dish. Add the potato mixture carefully and spread. Sprinkle with cheese. Bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for 25-30 minutes.