Slice the onion into thin 1/8-inch rings and set aside. Pat the pork chops dry with paper towels—this helps them brown better on the grill. In a small bowl, combine the Montreal Steak seasoning and smoked paprika, then generously season both sides of each pork chop. Let them sit at room temperature for 5-10 minutes while you prepare the other components. I like to season the meat early so the flavors penetrate the surface before cooking.
Toss the sliced onions in a bowl with the flour and Dad's All Purpose seasoning until evenly coated. Heat oil in a cast iron skillet on your grill to 375°F. Working in batches if needed, carefully add the floured onions to the hot oil and fry for 4-5 minutes, stirring occasionally, until they're golden brown and crispy. Remove with a slotted spoon and drain on paper towels. Keep them warm while you cook the pork chops.
Place the seasoned pork chops directly on a preheated grill at medium-high heat (about 375°F). Grill for 4-5 minutes on the first side without moving them, then flip and cook for another 4-5 minutes. Once both sides have a nice sear, brush generously with BBQ sauce on both sides, then continue cooking for 1-2 minutes per side until the internal temperature reaches 140°F (measured with a meat thermometer at the thickest part, away from the bone). The glaze will caramelize slightly, adding great flavor and a nice exterior.
Transfer the glazed pork chops to a clean plate and let them rest for 5 minutes—this allows the juices to redistribute throughout the meat, keeping it tender and juicy. While the pork rests, place the brioche buns cut-side down on the grill for 1-2 minutes until they're lightly toasted and golden. I prefer toasting buns on the grill for a subtle smoky flavor rather than in a toaster.
Spread 1 tablespoon of mayonnaise on the bottom bun of each sandwich. Top with a small handful of shredded lettuce, then place a rested pork chop on top. Pile a generous portion of the crispy fried onions from Step 2 onto each pork chop, then crown with the toasted top bun. Serve immediately while the pork is still warm and the buns are still crispy.