How to Make It
Preheat oven to 350°F. Pat both sides of slabs dry with paper towels. Using a sharp knife, remove thin membrane from back of each by slicing into it and pulling it off with a paper towel.
Make the Southern BBQ Dry Rub: Combine all ingredients in a medium bowl or large ziplock plastic bag. (You will have about 11/4 cups dry rub.) Divide Southern BBQ Rub evenly among slabs, rubbing on both sides of each (about 3/4 cup). Press gently to adhere. Place slabs, meaty side up, on a wire rack on an aluminum foil-lined baking sheet, and let stand at room temperature 30 minutes.
Bake in preheated oven until meat begins to pull away from bones but is not yet tender, about 1 hour and 30 minutes. Increase oven temperature to 450°F. (Do not remove ribs from oven.)
While the ribs are baking, make the Melting Pot BBQ Sauce. Heat a large (12-inch) cast-iron skillet over medium-high. Add tomatoes, and cook, stirring often, until skins begin to split and char, 5 to 7 minutes. Add onions and garlic, and cook, stirring often, 5 minutes. Add butter to pan, and cook until melted, about 1 minute. Add brown sugar; stir until combined. Cook, without stirring, until edges start to caramelize, 2 to 3 minutes. Stir in vinegar, mustard, Worcestershire, salt, and pepper; reduce heat to medium-low, and cook until tomatoes completely break down and mixture is slightly thickened, about 45 minutes.
Remove from heat, and carefully transfer to a blender. Remove center piece of blender lid (to allow steam to escape); process on high until almost smooth, about 1 minute, stopping to scrape down sides as needed. Cool completely, about 30 minutes. (You will have about 1 quart of sauce.)
Brush both sides of ribs with 1 cup Melting Pot BBQ Sauce. Bake until ribs are very tender and caramelized, 35 to 45 minutes, brushing with another cup barbecue sauce halfway through baking. Remove from oven, and let stand at least 10 minutes. Serve with remaining 2 cups barbecue sauce.