Cook the dry bulgur in four tablespoons of the butter, stirring, until the wheat is thoroughly coated. Cook the onion separately in the remaining butter until it is soft.
Combine the onion with the grains and stock in a covered casserole. Add salt and pepper to taste. Bake for thirty minutes; stir gently with a fork. Bake for fifteen minutes longer. At the end of the baking time, all of the liquid should be absorbed and the cracked wheat should be moist but fluffy.