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Preheat oven to 300 degrees F. Cut each rib rack into thirds or quarters. Place pieces in 10” x 13” rimmed baking pan.
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In a medium bowl whisk together the barbeque sauce ingredients: 1 cup chili sauce (reserve remaining ½ cup chili sauce for finishing), brown sugar, vinegar, Worcestershire sauce, mustard powder, salt, pepper, chili powder , onion and garlic.
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Pour the sauce over the ribs and turn them a couple of times to coat all sides. Arrange the ribs in one layer or slightly overlapping. Cover the dish tightly with foil paper and cook undisturbed for 2 hours.
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Remove pan from oven and increase oven temperature to 375 degrees F. Remove foil paper and baste ribs with the sauce and rearrange them if necessary. Leave uncovered and cook for another 30 minutes until the sauce starts to thicken and caramelize.
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Transfer the ribs to a serving platter and leave the sauce in the baking pan. Skim off the fat from the barbecue sauce and discard fat. Add remaining ½ cup chili sauce to the sauce and whisk to combine. Pour some of the sauce over the ribs on the platter and the remaining sauce into a bowl to serve with the ribs for extra dipping.