Melt butter; add onion, and cook until soft. Add remaining ingredients and cover. Simmer over low heat for 20 minutes, or until all liquid is absorbed, stirring occasionally. Turn off heat and allow standing for 15 to 20 minutes. Reheat to serve, if necessary. Variations: For Beef Pilaf, use beef stock in place of chicken stock, and add 1/2 teaspoon Worcestershire sauce. Parmesan Pilaf: Increase onion to 1/4 cup, and add 1/2 teaspoon garlic salt. When cooking is complete, stir in 1/3 cup Parmesan cheese and 1 tablespoon minced parsley.