Preheat oven to 350°. Place meat in roasting pan. Season with salt and pepper. Arrange onion rings on top of roast. Stir beer, chili sauce and garlic together in small bowl. Pour over meat.
Cover and roast until well-browned and tender (about 2 hrs). Baste with pan sauce several times during cooking.
Stir flour and water together to thicken sauce. Uncover roast and stir flour mixture into sauce. Bake 5-10 min longer. Serve immediately.