For flavorful ribs from the slow cooker, we cut the St. Louis-style ribs in half crosswise, coated each half liberally in a spice rub (a mix of paprika, brown sugar, salt, pepper, onion powder, and granulated garlic), arranged them on end (exposed rib side down) around the rim of the cooking insert, and let them slowly cook until tender. To get that signature shiny, sticky finish, we made an easy barbecue sauce that we brushed onto to the ribs before broiling them.
1. For the ribs: Combine paprika, sugar, salt, pepper, onion powder, and granulated garlic in bowl. Reserve 1 tablespoon spice rub for sauce. Pat ribs dry with paper towels and coat all over with remaining 5 tablespoons rub.
2. Arrange ribs vertically with thick ends pointing down and meaty side against interior wall of slow cooker (ribs will overlap). Cover and cook until ribs are just tender, 5 to 6 hours on high or 6 to 7 hours on low.
3. For the barbeque sauce: Meanwhile, whisk ketchup, apple juice, molasses, vinegar, Worcestershire, mustard, pepper, liquid smoke, and reserved 1 tablespoon spice rub together in medium saucepan. Bring to boil over medium heat, then reduce heat to medium-low and simmer, stirring occasionally, until thickened and reduced to 1 cup, about 10 minutes. (Sauce can be refrigerated for up to 3 days.)
4. Line rimmed baking sheet with aluminum foil and set wire rack in sheet. Using tongs, transfer ribs, meaty side up, to prepared rack. Let ribs sit for 10 minutes to allow surface to dry out.
5. Adjust oven rack 3 inches from broiler element and heat broiler. Liberally brush ribs with 1/2 cup sauce and broil until sauce is bubbling and beginning to char, about 4 minutes. Remove ribs from oven, brush with remaining 1/2 cup sauce, tent with foil, and let rest for 20 minutes. Cut ribs in between bones to separate. Serve.