How to Make It
In a large pot of boiling salted water, cook the noodles until al dente. Drain and rinse under cold water; drain again.
In a nonstick skillet, heat the oil. Add the onions and stir; cover and cook over moderate heat, stirring, until softened, about 10 minutes. Uncover and cook over moderately high heat, stirring frequently, until golden, about 8 minutes.
Add the mushrooms and cook, stirring, until softened and any liquid has evaporated, 6 minutes. Season with salt and pepper and transfer to a bowl.
In another bowl, stir the kasha with the egg. Add to the skillet and cook over moderate heat, stirring to break up any clumps, until lightly browned, 4 minutes. Add the stock and bring to a boil. Reduce the heat to low; simmer until the stock is absorbed and the kasha is tender, 10 minutes. Fold in the noodles and season with salt and pepper. Stir into the onion mixture and serve hot.