Sauté onion in hot oil in a large saucepan over medium-high heat 4 to 5 minutes or until tender. Add garlic; sauté 1 minute. Gradually stir in beer. Cook 8 to 10 minutes or until mixture is reduced by half. Reduce heat to medium.
Stir in barbecue sauce and next 2 ingredients, and cook 4 to 5 minutes or until thoroughly heated.
Note: We tested with Guinness Extra Stout beer and Sweet Baby Ray's Sweet 'n Spicy Barbecue Sauce.
Prep and cook sauce 3 days ahead. Cover and chill, and reheat when ready.