Trim mushrooms and wipe clean.
Heat butter in skillet, add mushrooms and onions; cook briskly for 4 minutes, stirring.
Sprinkle with salt and pepper, nutmeg, lemon juice, and flour.
Cook 1 to 2 minutes more.
Remove from heat; add sour cream and dill.
Super spread on pumpernickel.
May be used as filling in miniature tart shells.