Mix the liquid smoke into the barbecue sauce and set aside.
Layer onion in the bottom of the slower cooker. Layer the sliced apples over the onions.
Mix all the rub ingredients together. Rub all over the ribs.
Heat 2 tablespoons of the olive oil in a large skillet. Brown the ribs on all sides. You may have to work in batches. Add remaining olive oil to the skillet if needed.
To the slow cooker, add a layer of ribs (start with any that contain bones) on top of the apples and onions. Cover those ribs with about 1/3 cup of the barbecue sauce mixture, brushing on to cover the meat.
Layer remaining ribs on top of the first layer and pour on the rest of the barbecue sauce. Use a brush to ensure all ribs are covered.
Cover the slow cooker and cook on low for 7-8 hours or on high 4-5 hours.