In a small bowl, whisk together the smoked paprika, garlic powder, onion powder, salt, pepper, and brown sugar.
Season the country-style ribs on all sides with the dry rub mixture, pressing it into the meat.
Place the sliced onion wedges at the bottom of a 6-quart slow cooker.
Layer the seasoned pork ribs over the onions.
In another bowl, mix the BBQ sauce with the apple cider vinegar and pour it evenly over the ribs.
Cover and cook on the LOW setting for 7 to 8 hours (or HIGH for 4 hours) until the pork is fork-tender.
Optional: Carefully remove the ribs and place them on a baking sheet. Brush with extra sauce and broil for 3–5 minutes to caramelize the edges.
Serve the ribs with the softened slow-cooked onions and extra sauce from the pot.