Kasha with mushrooms (Kasha mit schveml)

kasha (roasted buckwheat groats) ((1 cup))

egg, lightly beaten

hot water ((2 cups))

grating of nutmeg

salt and freshly ground black pepper

butter

medium onions, thinly sliced

small white (button) mushrooms, thinly sliced ((9oz))

sour cream or smetana to serve