1. Preheat oven to 325 degrees Fahrenheit (170 degrees Celsius).
2. Rinse meat and pat dry. Lay onion rings on the bottom of a medium-sized roasting pan and place the brisket on top.
3. Combine olive oil, garlic, salt, and pepper. Smear evenly over the meat. Pour beer into the pan around the meat.
4. Combine all barbecue sauce ingredients in a small bowl and pour evenly over the brisket.
5. Cover well with aluminum foil. Bake for four hours.