Step 1In a saucepan, melt the butter over medium heat. Add the bacon and onion and cook, stirring with a wooden spoon, until the onion is golden brown, 6 to 8 minutes.
Step 2Stir in the cider vinegar and bring to a boil. Whisk in the brown sugar, mustard and hot sauce. Reduce the heat and gently simmer the sauce until mellow and richly flavored, 10 minutes. Correct the seasoning, adding salt, pepper and additional sugar or hot sauce as needed — the sauce should be sweet, tart, piquant and very flavorful. Cool to room temperature for serving. This makes about 2 cups sauce.
Step 1Cut the cabbage in half and remove the core. Cut the cabbage into 2-inch chunks, transfer to a food processor and finely chop — you should have about 4 cups. Transfer the cabbage to a mixing bowl.
Step 2Stir in enough of the barbecue sauce to make the slaw moist, but not so much that it becomes soupy. Add the celery seed. Correct the seasoning, adding additional sauce as needed and salt, pepper and additional celery seed to taste. The slaw tastes best served within a few hours of making.
Step 1Make the rub: In a small bowl, combine the salt, pepper, dry mustard and onion powder, and stir to mix. Generously season the pork shoulder on all sides with the rub. Store any extra rub in a lidded jar away from heat and light.
Step 2Set up your grill for indirect grilling and heat to medium. Brush or scrape the grill grate clean and oil it well.
Step 3Place the pork shoulder, fat-side up, on the grate over the drip pan. Add wood chunks or chips to the coals. You need 2 chunks or 1½ cups chips every hour. Indirect-grill the pork until it is fall-off-the-bone tender and the internal temperature reaches about 200 degrees, about 3 hours. (The cooking time will depend on the size of the Boston butt and the heat of the grill.) Replenish the charcoal as needed, roughly a half chimney every hour.
Step 4Transfer the cooked pork shoulder to a cutting board and rest for 10 minutes. Wearing insulated food gloves, break the pork shoulder into meaty chunks. Discard any skin, bones and large lumps of fat. Using meat claws or forks, tear the pork into meaty shreds, or finely chop it with a cleaver, or thinly slice with a knife.
Step 5Set up your grill for direct grilling. Brush the cut sides of the buns with melted butter and toast them on the grill, 1 to 2 minutes.
Step 6Pile the pulled pork onto the buns. Top each with a mound of Mustard slaw and a spoonful of Mustard brown sugar barbecue sauce. Add pickled red onions and pickle slices, if using, and dig in.