Prepare the ribs: Remove the thin membrane from the back of the ribs using a butter knife and paper towel for grip. Removing the membrane helps the seasoning penetrate better and keeps the ribs from turning chewy.Cut each rack into 2–3 sections so they fit easily in the slow cooker.
Season the ribs: In a small bowl, stir together the salt (2 teaspoons), pepper (1 teaspoon), brown sugar (1/4 cup), garlic powder (1 teaspoon), onion powder (1 teaspoon), ground mustard (1/2 teaspoon), and smoked paprika (▢2 teaspoons).Pat the ribs dry, then rub the seasoning mixture generously over all sides of the ribs.
Layer in the slow cooker: Arrange the ribs upright and slightly overlapping inside the slow cooker rather than laying them flat if possible. This helps them cook more evenly.
Prep Suace: Whisk together the Worcestershire sauce (1 tablespoon), apple cider vinegar (1 tablespoon), and 1/2 cup barbecue sauce. Brush some of the sauce mixture over the ribs.
Cook: Cover and cook on LOW for 7–8 hours, or until the ribs are very tender but still hold together.
Finish under the broiler: Carefully transfer the cooked ribs to a foil-lined baking sheet.Brush generously with additional barbecue sauce.Broil 3–5 minutes, or until the sauce becomes sticky, bubbling, and lightly caramelized. Watch carefully so the sugars do not burn.For an extra lacquered finish, brush with another layer of sauce and broil 1–2 minutes more.
Serve: Let the ribs rest for a few minutes, then slice and serve warm with extra barbecue sauce if desired.
Notes
Use LOW heat for the best texture. LOW heat gives the ribs time to become tender without drying out or falling apart too aggressively. HIGH works in a pinch, but LOW gives better results.
Stand the ribs up if possible. Arranging the ribs upright around the edges of the slow cooker helps them cook more evenly and keeps them from sitting fully submerged in liquid.
Broil twice for extra sticky ribs. For ribs that look glossy and irresistible, brush with sauce, broil, brush again, and broil briefly a second time.
Let them rest before slicing. A few minutes of resting helps the sauce settle and makes the ribs easier to cut and serve.