Preparation

• In a spice grinder, combine paprika, brown sugar, salt, mustard seed, garlic powder, onion powder, oregano, coriander seed, cumin seed, red pepper flakes, and black pepper and reduce to a fine powder, working in batches as needed.
• In a medium saucepan, whisk together 3 tablespoons spice rub, grated onion, ketchup, mustard, molasses, Worcestershire sauce, vinegar, and liquid smoke. Bring to a bare simmer over medium-low heat and cook until reduced and thickened, about 20 minutes. Set aside.
• Remove the papery membrane on the back of the ribs, using a paper towel or kitchen towel to grip and pull it away in one piece. Rub racks of ribs generously on all sides with about 1 cup of the spice rub mixture. (Set aside 3 tablespoons of spice rub if making dry-style ribs.)
• Sprinkle ribs all over with 1 teaspoon liquid smoke, then rub in with hands for even coverage. If ribs don't smell smoky enough, sparingly rub in more liquid smoke. Wrap each rib rack in plastic and refrigerate for at least 2 hours and up to 8.
• Preheat oven to 250°F (120°C) and adjust rack to middle position. Set a wire rack inside a rimmed baking sheet. Remove and discard plastic from the rib racks, then wrap each rib rack in foil, sealing it tightly, and set on prepared baking sheet. Bake ribs for 2 hours. Remove and discard foil. Return ribs to oven, meaty side up, and continue to cook until a toothpick or skewer can be pushed into meat with minimal resistance, about 1 ½ hours longer.
• Increase oven temperature to 500°F (260°C). Meanwhile, rub racks with 3 tablespoons spice rub (if making them dry-style) or brush with sauce (if using). Return to oven and cook, meaty side up, until well browned, 5 to 8 minutes. Divide ribs between bones, if desired; serve, passing additional sauce at the table, if using.