This, like most mushroom dishes, is very popular in Poland, and is served with meat, or as an hors-d’œuvre.
Brown the chopped onion in the butter, sprinkle with flour, brown and add the milk gradually. Bring to the boil and add the sliced mushrooms. Season with salt, pepper and paprika, simmer, and add half the sour cream and cook very gently till the mushrooms are tender. Before serving, stir in the remaining sour cream.