If your butcher has not already done so, remove the thin papery membrane from the bone-side of the ribs by working the tip of a butter knife or a screwdriver underneath the membrane over a middle bone. Use paper towels to get a firm grip, then tear the membrane off.
Lay the ribs flat, meat side down, in a 9- by 13-inch glass baking dish.
For the brine, in a large mixing bowl, combine the water, celery, lemon - squeeze the lemon quarters but toss the rind into the brine as well - onion, kosher salt, brown sugar, celery salt, Worcestershire sauce, peppercorns, and garlic powder. Whisk until the salt and sugar crystals dissolve. Pour over the ribs. Cover and refrigerate for 6 to 8 hours, or even overnight, turning once or twice.
Drain the ribs, knocking any solids off the meat, and dry thoroughly with paper towels. Discard the brine. Slather both sides of each rack of ribs with the mustard.
When ready to cook, start the Traeger grill on Smoke with the lid open until the fire is established (4 to 5 minutes). Put the ribs directly on the grill grate and smoke the ribs, meat-side up, for 2 hours.
Season generously on both sides with Traeger's Sweet Rub. Tear off a large sheet of heavy duty foil - approximately 24 by 18 inches - and lay the racks of ribs on top of each other, meat side up. Pull up the sides of the foil and add the beer. Fold the edges of the foil so the meat is tightly enclosed.
Increase the temperature of the Traeger to 300. Put the foiled ribs directly on the grill grate and continue to cook for 1-1/2 to 2 hours. Carefully open the package - hot steam will escape - and check the ribs: They should be quite tender when you insert a skewer or knife between the middle bones. If not, refold the foil and continue to cook until they are tender.
In the meantime, make the sauce: Melt the butter in a small saucepan. (You can do this on the grill.) Whisk in the hot sauce.
Remove the ribs from the foil and let rest for 3 to 5 minutes. Cut into individual ribs and arrange on a large platter or in a shallow bowl. Pour the sauce over the ribs, and using tongs, turn the ribs to coat. Serve at once with the blue cheese dressing on the side.