Place the kasha in a heavy saucepan and add the beaten egg. Stir well, so each grain is well coated, then cook over a gentle heat for 5 minutes or until the egg is set.
Pour in the hot water and season with nutmeg, salt and pepper. Cover and simmer for 15 minutes, or until the kasha is tender and the liquid has been absorbed.
Meanwhile, melt the butter in a large frying pan and add the onions. Cook over a gentle heat until they start to turn golden. Add the mushrooms and continue to cook until they are tender and any liquid has evaporated.
Stir the onions and mushrooms into the cooked kasha and simmer for 1–2 minutes.
Serve at once, with sour cream on the side.
