Melt the butter in a large saucepan. Add the onions and cook over medium heat for 10 minutes, stirring occasionally, until tender but not browned. Add the chicken stock, 1 1/2 teaspoons salt, and 1/2 teaspoon pepper and bring to a full boil. Stir in the couscous, turn off the heat, cover, and allow to steam for 10 minutes. Fluff with a fork and serve hot.