Step 1Heat the oven to 375 degrees.
Step 2Arrange the ribs, rounded side up, in a single layer on a rack in a shallow roasting pan. Roast, uncovered, 30 minutes. Combine the chili powder, 2 teaspoons brown sugar, salt and pepper to taste in a small dish. Remove the ribs from the oven and sprinkle the chili mixture on both sides, rubbing the mixture in. Continue to roast the ribs, rounded side down, for 20 minutes. Use tongs to turn. Roast 10 minutes longer.
Step 3Meanwhile, bring the barbecue sauce, onion, water, pureed chipotles, 1 to 2 tablespoons brown sugar or lemon juice and cumin to a boil; reduce the temperature and simmer, uncovered, 10 minutes.
Step 4After the ribs are cooked, cut into 2-rib portions; place in a large shallow casserole. While the ribs and barbecue sauce are hot, pour all but 1/2 cup of the barbecue sauce over the ribs. Brush the sauce on both sides. (This can be made a day ahead, cooled and refrigerated, well covered. Chill reserved sauce separately.) Bake the ribs, rounded side up, covered with foil, at 350 degrees for 30 minutes. Uncover; bake about 20 to 30 minutes longer. Serve hot; pass the remaining sauce separately.