How to Make It
Preheat oven to 275°F with rack in middle position. Stir together salt, brown sugar, smoked paprika, pepper, garlic powder, and dry mustard in a bowl. Remove thin outer membrane on back of ribs by loosening with a butter knife and pulling it off; discard. Brush slabs evenly with oil; sprinkle with spice mixture, gently patting to adhere. (If desired, wrap ribs with plastic wrap; place on a baking sheet, and chill at least 4 hours or up to 24 hours.)
Line a baking sheet with aluminum foil; set a wire rack inside sheet. (If you wrapped and chilled the ribs, remove and discard plastic wrap.) Wrap each slab with heavy-duty aluminum foil; place on prepared wire rack. Bake in preheated oven until tender, 2 1/2 to 3 hours. Remove from oven, and unwrap ribs; discard foil and drippings. Place ribs directly on wire rack; brush 1/2 cup barbecue sauce on top of each slab. Return to oven; bake at 275°F until ribs are tender and meat is pulling away from bones, 45 minutes to 1 hour.
Remove from oven; brush slabs evenly with remaining barbecue sauce. Increase oven temperature to broil, and return ribs to middle rack. Broil until sauce bubbles and starts to caramelize, 3 to 5 minutes. Remove from oven; let rest 5 minutes. Serve.