Sauté the onion in the butter in a heavy casserole until tender and transparent but not browned.
Add the rice and continue to cook over low heat, stirring occasionally, until rice grains turn opaque. Do not allow rice to brown.
Add the stock and season with salt and pepper to taste. Cover tightly and bake for five minutes. Reduce the heat to 350°F. and bake for fifteen to twenty minutes longer, or until rice has absorbed all the stock.
Mushroom pilaff: Sauté one-half pound mushrooms, sliced, in one-quarter cup butter, and add to pilaff before serving.
Nut pilaff: Add one-quarter cup toasted cashew nuts to pilaff before serving.