Note: I prefer to make this topping on a griddle insert on my grill since I'm already outside smoking the sausage. You can also make it in a large skillet on your stovetop.
Heat a large skillet over high heat.
Add onions and peppers and saute until tender and golden brown, about 6 minutes.
Add the garlic and saute for 3 minutes.
Stir in the tomato paste, beer, and BBQ sauce.
Cook for 5 minutes then reduce the heat to a simmer and cook another 10 minutes, stirring occasionally. The sauce should be nice and thick. If it gets too thick add a bit more beer or some water.