Trim brisket of excess fat and apply mustard all over as a binder.

Season generously with a pepper-forward brisket rub and refrigerate for 1-2 hours.

Smoke on pellet smoker at 210 degrees with hickory pellets, water pan underneath, for 9-10 hours until 165-170 degrees.

Wrap in butcher's paper with apple cider vinegar and beef tallow, then smoke at 250 degrees until 204-206 degrees.

Rest wrapped brisket in cooler for 2-3 hours.

Sauté diced onions with avocado oil, kosher salt, and butter until caramelized.

Assemble melt with sourdough bread, fried onions, Colby jack cheese, brisket slices, caramelized onions, BBQ sauce, provolone cheese, and top bun.

Toast in cast iron skillet with butter for 3-4 minutes, flipping once.