For the ribs: Prepare an outdoor grill l for cooking over indirect medium heat (if your grill has a thermometer, aim to keep the temperature at about 350 degrees F). Combine the brown sugar, chili powder, smoked paprika, granulated garlic and 1 teaspoon salt and 1/4 teaspoon pepper in a small bowl.
Insert a butter knife between the membrane and bone on the back of the rack of ribs. Grab the membrane with a paper towel to grip it and pull off and discard the membrane. Sprinkle the spice mixture on both sides of the ribs and rub it in. Wrap the ribs tightly in aluminum foil and set on the indirect side of the grill. Cook, turning the package once, until the meat is tender and pulling from the bone but not totally falling apart, 1 hour 15 minutes to 1 hour 30 minutes.
For the BBQ sauce: Meanwhile, combine the ketchup, vinegar, light brown sugar, honey mustard, hot sauce, chili powder, smoked paprika, granulated garlic, liquid smoke if using and 1/4 cup water in a small saucepan. Bring to a simmer and cook over medium heat, stirring occasionally, until thickened, 10 to 12 minutes.
When the ribs are tender, remove them from the grill and unwrap them (leave the grill on). Let cool until you can touch the ribs without burning your fingers. With a knife, slit the skin on the back of the ribs and carefully pull out the bones, keeping the slab in one piece.
Increase the grill temperature to high for cooking over direct heat. Add the boneless slab to the grill, rounded-side up, and brush with some of the BBQ sauce. Grill until the underside begins to crisp, about 2 minutes. Carefully flip the slab and brush the underside of the ribs with more sauce. Continue to grill the ribs until they're glossy and caramelized, 2 to 3 minutes more. Place the ribs on the hero roll, brush with more sauce and top with onions and pickles. Cut into 4 pieces. Serve with the extra sauce on the side.
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