How to Make It
Fill a small aluminum pan with water, and place on grate inside an electric pellet grill. Fill pellet container with hardwood pellets according to manufacturer’s instructions. Set temperature to 225°F, close lid, and preheat pellet grill 10 to 15 minutes.
While grill preheats, stir together sugar, paprika, cumin, and garlic powder in a small bowl. Slide a paring knife underneath membrane on back side of each rack, starting at 1 end. Lift knife up and down to loosen membrane. Using a paper towel, grab membrane, and slowly pull off; discard. Place ribs on a rimmed baking sheet; sprinkle both sides with salt. Brush 1/2 cup of the mustard in an even layer on both sides; sprinkle with spice mixture. Whisk together vinegar, water, and remaining 2 tablespoons mustard in a small bowl.
Place ribs on grill grate, close lid, and grill, basting every 45 minutes with vinegar mixture, until meat is tender to the touch and falling away from bones on the ends, 5 to 6 hours, flipping ribs halfway through. Transfer ribs to a cutting board; let rest 20 minutes. Cut ribs between the bones, and serve.