In a large, deep skillet, heat the remaining 3 tablespoons of butter. Add the onion and cook over moderately high heat, stirring occasionally, until softened, about 5 minutes. Add the chopped mushrooms and cook for 2 minutes, until sizzling. Stir in the steamed rice. Add the remaining 1/4 cup of sake, season with salt and pepper and cook until the sake is absorbed, about 1 minute. Add 1/2 cup of the mushroom stock and cook over moderate heat, stirring, until absorbed. Add another 1/2 cup of the stock and cook, stirring, until the rice is tender but not mushy, about 10 minutes; you will have stock left over for another use. Stir in the chives and serve right away.