Combine 1 tablespoon brown sugar, 2 teaspoons paprika, 1 teaspoon garlic powder, 1 teaspoon onion powder, 1 teaspoon salt, and ½ teaspoon ground black pepper in a small bowl.
Rub generously over a 3-4 pounds flat beef brisket and set it aside to rest for 30 minutes, or cover it with foil and let it rest in the refrigerator for up to 24 hours.
Place the rubbed brisket in the bottom of a 6 or 7-quart slow cooker. Place 1 small quartered onion into the slow cooker, then top it all off with 1 cup homemade BBQ sauce.
Cook on LOW for 8-10 hours, or on HIGH for 5-6 hours. Once the brisket is tender, you may crisp it by placing it on a baking sheet and roasting it in the oven for about 5 minutes with the broiler on high. Allow it to rest for 10 minutes before serving.