Slice Onion (4) into thick rings. Sauté the onions in Butter (4 tablespoon) until softened, but not completely cooked. Do not caramelize.
In a mixing bowl, combine Barbecue Sauce (1/2 cup), Dijon Mustard (1/4 cup), and Hot Sauce (1 teaspoon). Mix well.
Add onions to Crockpot. Pour barbecue sauce mixture over onions and mix well.
Grill Bratwurst (12) about 3 minutes on each side to crisp the outer skin.
Add brats to Crockpot. Pour Beer (24 fluid ounce) over onions and brats. Cook in Crockpot 6 to 7 hours on low.