Stir the lemon juice into the broth, bring to a boil, then add the finely chopped mushrooms; simmer over low heat for about 8-10 minutes, until the mushrooms soften.
Meanwhile, in another saucepan, melt the butter, add the flour, and cook to a golden roux.
Mix the roux smooth with some of the mushroom broth, then pour the whole thing back into the mushroom base.
Bring to a boil and cook until the sauce thickens.
Season with salt, stir in the sour cream, and warm through again (but do not let it boil).
Based on: Malatinszky Fanny szakácskönyve (a classic Hungarian cookbook), modernised. Calories and protein are estimated.