Thaw the plaice fillets. Put rice and 1/2 teaspoon of salt in 1/2 litre of cold water, bring to the boil and allow to swell for 15 minutes at low heat. Peel and finely chop the onion.
Clean, wash and quarter the mushrooms. Heat the fat in a pan. Sauté onion and mushrooms in it. Add crayfish soup paste with 1/2 litre water. Bring to the boil. Wrap the plaice fillets once and put them into the sauce.
Cover and cook over a low heat for about 8 minutes. Wash and chop the dill and sprinkle over the top. Garnish with lemon wedges.