In a small bowl, mix together brown sugar, salt, paprika, garlic powder, black pepper, and mustard powder. Rub this dry seasoning all over the ribs.
Spread chopped onion evenly in a 6-quart slow cooker. Add water.
Place the ribs in the slow cooker (you can stack them in two layers) and brush half of the BBQ sauce over them.
Cover and cook on LOW for 8-10 hours or on HIGH for 4-6 hours.
(Optional) For crispy, charred edges, transfer ribs to a baking sheet lined with parchment paper, brush with remaining BBQ sauce, and broil on high for 5-7 minutes. Serve immediately.
Notes
Nutritional Information per serving: Calories: 480 | Total Carbs: 18g | Net Carbs: 17g | Fats: 28g | Protein: 38g