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1
Working in batches, combine paprika, brown sugar, salt, mustard seed, garlic powder, onion powder, oregano, coriander seed, cumin seed, red pepper flakes, and black pepper in a spice grinder and reduce to a fine powder.
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2
Combine 3 tablespoons spice rub, grated onion, ketchup, mustard, molasses, Worcestershire sauce, vinegar, and liquid smoke in a medium saucepan and whisk thoroughly. Bring to a bare simmer and cook until reduced and thickened, about 20 minutes. Set aside.
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3
Remove the papery membrane on the back of the ribs, using a paper towel or kitchen towel to grip it, pulling it away in one piece. Rub ribs generously on all sides with spice rub mixture. (Set aside 3 tablespoons of spice rub if making dry-style ribs.)
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4
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 and up to 8 hours.
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5
Preheat oven to 250u00b0F (120u00b0C) and set rack in center position; set a wire rack in 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 to oven, meaty side up, and continue to cook until a toothpick or skewer can be pushed into meat with minimal resistance, about 1 1/2 hours longer. (Cooking times can vary quite a bit depending on the exact size of rib racks and the oven being used.)
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6
Increase oven to 500u00b0F (260u00b0C). Meanwhile, rub racks with remaining 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 10 minutes. Divide ribs between bones, if desired; serve, passing additional sauce at the table, if using.
