Clean and wash the leek. Peel, wash and thinly slice the carrots. Cut both vegetables into fine 4 cm long strips. Peel and wash the potatoes and cook in salted water for about 20 minutes. In the meantime wash the plaice fillets and dab them dry.
Place on a work board with the skin side facing upwards. Cut the salmon into strips of about 3 cm width and lay them on the fillets. Roll up each fillet loosely and pin it with a wooden skewer. Melt butter in a pan. Add vegetables and fish and deglaze with 150 ml water and white wine. Season with salt and pepper, cover and cook over medium heat for 6-8 minutes. Keep fish and vegetables warm. Strain the stock, bring to the boil and stir in the sauce thickener. Let it boil for about 1 minute. Stir in mustard and sour cream and season with salt and pepper.
Keep fish and vegetables warm. Strain the stock, bring to the boil and stir in the sauce thickener. Let it boil for about 1 minute. Stir in mustard and sour cream and season with salt and pepper. Wash the dill, dab dry and cut finely. Drain potatoes and sprinkle with dill. Arrange plaice rolls on a bed of vegetables and serve with potatoes and sauce