Remove the papery membrane on the back of the ribs using a paper towel or kitchen towel to grip it and pull it away in one piece.

Divide each rack of ribs into three to four portions with three to four ribs each by cutting through the meat in between the ribs.

Working in batches, combine the paprika, brown sugar, salt, mustard seed, black pepper, garlic powder, oregano, coriander seed, and red pepper flakes in a spice grinder and reduce to a fine powder.

Set aside 3 tablespoons / 45 ml of spice rub mixture.

Rub ribs generously on all sides with the remaining spice rub mixture.

Place individual portions of rubbed ribs in vacuum bags.

Add 4 drops (about ⅛th teaspoon / 1 ml) of liquid smoke to each bag.

Seal the bags, transfer them to the refrigerator, and let them rest for 4 to 12 hours.

Set your Precision Cooker to 145°F / 63°C for extra-meaty ribs or 165°F / 74°C for more traditionally textured ribs.

Add ribs to the water bath and cover with a lid, aluminum foil, or ping pong balls. Cook for 36 hours at 145°F / 63ºC or 12 hours at 165°F / 74ºC.

Transfer cooked ribs to a large bowl of water filled with ice to chill thoroughly.

To make the sauce, combine remaining 3 tablespoons spice rub, grated onion, ketchup, mustard, molasses, Worcestershire sauce, vinegar, and remaining liquid smoke in a medium saucepan and whisk to combine. Bring to a bare simmer and cook until reduced and thickened, about 20 minutes. Set aside.

Remove ribs from vacuum bags and carefully pat dry with paper towels.

Adjust oven racks to upper and lower middle positions and preheat oven to 300°F / 150°C. Line two rimmed baking sheets with aluminum foil and place a wire rack in each. Divide ribs evenly on racks, facing up. Transfer ribs to oven and cook until surface is sizzling and ribs are heated through, about 20 minutes.

Brush ribs with sauce and return to oven for 10 minutes. Remove from oven, brush with another layer of sauce, and return to oven until it is dried and sticky, about 10 minutes longer.

Remove ribs from oven, paint with one last layer of sauce, and serve, passing extra sauce at the table.