How to Make It
Light a grill. In a medium skillet, heat the oil. Add the onion and garlic and cook over moderate heat until tender, about 4 minutes. Reduce the heat to low and stir in the ketchup, chipotle, Worcestershire, dry mustard and 1 tablespoon of the vinegar and cook for 1 minute, until warm. Transfer to a blender and puree until smooth. Scrape the barbecue sauce into a bowl.
In a medium bowl, whisk the mayonnaise with the remaining 1 tablespoon of cider vinegar and season with salt and pepper. Add the coleslaw mix, cilantro and scallion and toss well.
Season the salmon with salt and pepper and grill skin side up over moderately high heat for 3 minutes. Turn and brush with the barbecue sauce. Grill until the skin is crisp and the fish is just cooked through, 4 minutes longer. Toast the buns on the grill.
Transfer the salmon to the buns and top with the slaw. Serve, passing the remaining barbecue sauce at the table.