How to Make It
Prepare the Barbecue Sauce: Heat oil in a medium saucepan over medium-high. Add onion, garlic, and jalapeño; cook, stirring often, until lightly browned around edges, about 8 minutes. Stir in tomato puree, vinegar, ketchup, brown sugar, molasses, Worcestershire sauce, and liquid smoke. Reduce heat to medium-low; cook, stirring occasionally (making sure sauce doesn’t stick or burn), until thickened and reduced to 2 cups, about 1 hour. Remove from heat; cool to room temperature, about 30 minutes. Chill until ready to use.
Prepare the Ribs: Preheat oven to 275°F, and place oven rack in middle of oven. Combine brown sugar, chili powder, salt, paprika, onion powder, garlic powder, dry mustard, and black pepper in a medium bowl; set aside. Remove the thin outer membrane on back of ribs by loosening with a butter knife and pulling it off; discard. Brush each side of both slabs evenly with Dijon mustard; sprinkle liberally with spice mixture, patting gently to adhere. Wrap each slab individually in aluminum foil (or chill uncovered up to 12 hours).
Place wrapped ribs on a wire rack set on a sheet tray lined with aluminum foil. Bake in preheated oven until tender, 2 to 2 1/2 hours. (Check for doneness by removing ribs from oven, unwrapping foil, and picking up 1 end with tongs. The ribs are ready when they bend and meat on top is tender and easily pulls apart.) Unwrap ribs; discard foil and drippings. Place directly on wire rack, and spread 1/2 cup of the Barbecue Sauce on top of each slab. Return to oven, and cook at 275°F for 30 minutes.
Brush slabs again with some of the Barbecue Sauce. Adjust oven to broil, and return to middle rack. Broil until sauce bubbles and starts to caramelize, 3 to 5 minutes. Remove from oven, and let rest 5 minutes before slicing. Serve with remaining sauce.