Pat the ribs dry with paper towels. Cut a full rack into individual ribs or smaller sections if needed to fit the slow cooker.
Combine the salt, black pepper, smoked paprika, garlic powder, onion powder, and chili powder in a small bowl. Season all sides of the ribs.
Heat the olive oil in a large skillet over medium-high heat. Brown the ribs in batches for about two minutes on each side. Transfer to a plate.
In a medium bowl, combine one cup of barbecue sauce, ketchup, brown sugar, apple cider vinegar, Worcestershire sauce, garlic, Dijon mustard, and optional liquid smoke.
Spread the sliced onion over the bottom of a 6-quart slow cooker. Arrange the ribs over the onion.
Pour the barbecue mixture over the beef, spreading it over the ribs.
Cover and cook on LOW for seven to eight hours or until the beef is fork-tender and pulls easily away from the bone.
Carefully transfer the ribs to a foil-lined baking sheet.
Brush the ribs with the remaining ½ cup of barbecue sauce.
Broil for two to four minutes, watching closely, until the sauce is bubbling and caramelized around the edges.
Brush with additional barbecue sauce if desired and serve hot.