Sprinkle roast with garlic salt and pepper. Place roast fat side up in a shallow roasting pan. In a skillet, saute onion in butter until tender; stir in the remaining ingredients. Stir until heated. Pour over roast.
Bake, uncovered, at 350° for 1 hour or until a thermometer reads 160°. Remove roast to a serving platter and keep warm. Let stand for 10 minutes.
Pour drippings and loosened brown bits into a bowl. Skim fat. In a small saucepan, combine flour and water until smooth. Gradually stir in drippings. Bring to a boil; cook and stir for 2 minutes or until thickened. Slice roast and serve with gravy.