Preparation

• Position a rack in the center of the oven and heat the oven to 300°F. Sprinkle and press 1/4 cup of the rub on both sides of each rib rack. Put the racks, meaty side up, on a broiling pan or a wire roasting rack set over a baking sheet. Lightly season the ribs with salt and put them in the oven. After the first hour,  rotate the pan every 30 minutes (Note: If you use two baking sheets, switch their position in the oven, too). The ribs will sizzle gently as they cook, and they’ll become tender after about 2 hours in the oven.
• To test for doneness, pick up the center of the ribs with tongs; the ends of the ribs should flop downward (this means the fat and cartilage have broken down), and a skewer inserted between the ribs should meet little resistance. If the meat between the ribs is still tough, keep cooking, checking every 15 minutes and rotating the pan.
• To make the sauce, heat the oil and add the onion to a medium sauce pan over medium heat. Season lightly with salt and sauté until translucent, about 3 minutes. Add the tomato purée, brown sugar, tomato paste, cider vinegar, Worcestershire sauce, Tabasco, mustard, chili powder, paprika, and cayenne and stir well. Turn down the heat to low and let the sauce simmer until it thickens slightly, about 30 minutes. Remove from the heat, season to taste with salt, and let cool to room temperature. The sauce will keep for about a week in the refrigerator.  
• Remove the rib racks from the oven, put them on a cutting board meaty side down (so they’re easier to slice), and slice them into individual ribs. Arrange the ribs on a platter and serve with the sauce on the side.