Balsamic-Beer Grilled Flank Steak

1 1/2 lb flank marinating steak
1/2 cup beer (either ale or lager)
1 small onion finely chopped
2 garlic cloves minced
1/4 cup ketchup
1 tbsp chili powder
2 tbsp balsamic vinegar
2 tbsp brown sugar
1 1/2 tbsp grainy or dijon mustard
1/4 tsp each salt and pepper

Pierce steak several times with fork; place in resealable plastic bag or in shallow dish. Combine remaining ingredients; mix well and pour over meat. Seal bag tightly or cover dish. Let meat marinate in refrigerator, turning occasionally for at least 12 hours. Remove meat from marinade and grill over medium-high heat for 5 to 6 minutes on each side for medium doneness. Remove from heat and allow to rest 5 minutes. Transfer marinade to small saucepan and boil 5 to 10 minutes until slightly thickened.
To serve, slice steak in thin strips across the grain and top with sauce if desired.