Whisk the tomato paste, brown sugar, vinegar, chili powder, mustard, garlic, paprika, Worcestershire, 1 1/2 tablespoons salt and 1 teaspoon pepper in the insert of a 6-quart slow cooker until smooth. Cut each rack of ribs in half and add to the slow cooker. Use your hands to rub the sauce on to the ribs. Cover with a lid and cook on low heat until tender, about 4 hours.
Gently transfer the ribs to a baking sheet and cool for at least 20 minutes; this will help maintain the shape of the ribs as they grill. Whisk the sauce at the bottom of the slow cooker until smooth. Reserve 1/4 cup of the sauce to brush on to the ribs and save any remaining sauce to serve on the side (it will thicken as it cools).
Preheat a grill or grill pan to medium-high heat. Lightly oil the grates. Grill the ribs, top-side down, until lightly charred in parts, 3 to 5 minutes. Flip, brush the top side of the ribs with the reserved barbecue sauce and grill until the underside of the ribs are lightly charred in parts, 3 to 5 minutes. Remove to a platter and serve immediately with the extra barbecue sauce on the side.
Alternatively, broil the ribs to char. Preheat the broiler with a rack set 4-inches from the heat source. Transfer the cooked ribs to a foil-lined baking sheet. Brush the top of the ribs with the reserved sauce and broil top-side up, until browned and bubbling, 3 to 5 minutes.
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