Warm up the stock and keep it simmering.
Beat the egg lightly and mix with the kasha so that the grains are well coated.
Pour kasha into the pan you just used for the onions. Cook over medium-high heat for 2-3 minutes, stirring, to break apart any clumps.
Add the boiling stock, cover, and simmer until the stock is absorbed and the kasha is cooked (10-12 minutes).