Spray 6-quart slow cooker with cooking spray. Layer onion slices in slow cooker. In small bowl, stir chili powder, 1 teaspoon cumin, the paprika, 1 tablespoon salt and the pepper until blended. Rub into brisket; place in slow cooker. Sprinkle any remaining rub on top of meat. In small bowl, stir brown sugar, ketchup, water and cider vinegar until blended. Pour over brisket.