Peel and wash the potatoes, cut them into pieces and boil them in salted water for about 25 minutes. Wash the herbs, dab dry and chop the leaves finely, except for a dill stalk. Wash the lemon, rub dry and grate the peel thinly. Mix the herbs, lemon zest, 1 tablespoon butter and a little salt. Wash the plaice fillets, dab dry, season on both sides and place on a board with the skin side facing upwards. Spread herb butter on the fillets, roll them up from the head side and pin them with a wooden skewer.
Peel onion and cut into fine cubes. Melt 1 teaspoon of butter in a saucepan and fry the onion in it. Add 50 ml white wine and 50 ml broth. Put the rolls into the pot and let them simmer covered at low heat for about 8 minutes. Heat 30 g butter in a small pot. Add flour and sauté. Add 50 ml white wine, 150 ml stock, horseradish and Alpro Soya, stirring continuously. Bring to the boil. Bring milk to the boil. Drain potatoes, mash and stir in the milk. Season the mashed potatoes with salt, pepper and a little grated nutmeg.
Add flour and sauté. Add 50 ml white wine, 150 ml stock, horseradish and Alpro Soya, stirring continuously. Bring to the boil. Bring milk to the boil. Drain potatoes, mash and stir in the milk. Season the mashed potatoes with salt, pepper and a little grated nutmeg. Take the rolls out of the pot and keep them warm. Add fish stock and prawns to the sauce and bring to the boil again