Use a fork to prick several holes in the skin of each bratwurst. In a saucepan combine bratwursts, beer, and peppercorns. Bring to boiling; reduce heat. Cover; simmer about 20 minutes or until bratwursts are no longer pink. Drain.
Meanwhile, in a small skillet cook onion in hot margarine until tender but not brown. Add caraway seed and cook 5 minutes more. Stir in the vinegar and the Worcestershire sauce.
Grill bratwursts on an uncovered grill directly over medium-hot coals for 7 to 8 minutes or until skins are golden, turning frequently (170 degrees F). Serve bratwursts on buns with mustard. Spoon onion mixture over bratwursts. Makes 4 servings.