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.
To make the Southern BBQ Dry Rub: combine all ingredients in a medium. Divide Southern BBQ Rub among the slabs, rubbing on both sides of each (about ¾ cup). Place slabs, meaty side up, on a wire rack on an aluminum foil-lined baking sheet, and let stand at room temperature 30 mins.
Bake in preheated oven until meat begins to pull away from bones but is not yet tender, about 1½ hr. Increase oven temperature to 450°F.
While the ribs are baking, make the Melting Pot BBQ Sauce. Heat a large cast-iron skillet over medium-high. Add tomatoes, and cook, stirring often, until skins begin to split and char, 5 to 7 mins. Reserve a few of the charred tomatoes to serve. Add onions and garlic, and cook, stirring often, 5 minutes. Add butter to pan and when melted, add brown sugar; stir until combined. Cook, without stirring, until edges start to caramelize, 2 to 3 mins. Stir in vinegar, mustard, Worcestershire, salt, and pepper; reduce heat to medium-low, and cook until tomatoes break down and mixture is slightly thickened, about 45 mins.
Remove from heat and transfer to a blender. Remove center piece of blender lid to allow steam to escape; process on high until almost smooth. Cool completely, about 30 mins.
Brush both sides of ribs with 1 cup Melting Pot BBQ Sauce. Bake until ribs are very tender and caramelized, 35 to 45 mins, brushing with another cup BBQ sauce halfway through baking
Remove from oven, and let stand at least 10 mins. Serve with remaining 2 cups BBQ sauce and reserved charred tomatoes.