Sprinkle salt, pepper and garlic salt to taste on both sides of roast. Place roast in bag. In a small bowl combine first nine ingredients. Mix well.
Combine cornstarch and water and stir until smooth. Pour over roast and top with sliced onions. Tie bag and turn roast upside down and shake so seasoning covers all while holding sliced onion on top. Return to pan, being sure to move onion back to top of roast.
Poke about 5 small holes in top of bag and bake at 350 for 2 hours or until top is a bit brown and roast is tender.