Light a grill or smoker to medium heat (about 300°F).

Make the mop sauce: Whisk together mustard, molasses and beer in a medium bowl until smooth.

Check to see if the membrane has been removed from the backside of the ribs. If not, insert a blunt dinner knife under one edge of the membrane and use a paper towel to grab the edge and pull the membrane off the ribs. It should come off in one piece.

Season ribs with salt and pepper.

Layer 2 (18-inch) sheets of foil on a work surface and set a rack of ribs in the center; pull the edges of the foil up around ribs.

Brush both sides of ribs with a generous slather of the mop sauce, then seal tightly in the foil. Repeat with the remaining rack.

Grill rib packets over moderate heat, covered, for about 45 minutes.

Using tongs, transfer rib packets to a baking sheet and let cool slightly.

Meanwhile, transfer the remaining mop sauce to a small saucepan and bring to a boil.

Reduce heat and simmer, stirring, until thick and syrupy, 10 to 12 minutes; let cool completely.

If you're using a smoker, add wood chunks to the periphery of the hot coals, and vent to maintain a smoking temperature of 225°F to 250°F.

If you’re using a conventional grill, soak wood chips in water for 30 minutes, then remove the grill grate; turn the heat off on half of the grill or rake the coals to one side.

Scatter wood chips directly on the flames or coals and replace the grill grate.

When chips are smoking, carefully open the rib packets and arrange the racks on the grill over indirect heat.

Discard the foil and juices.

Cover the grill and smoke until ribs are very tender, 45 minutes to an hour.

Brush ribs with the reduced mop sauce, transfer to a platter and serve.