Prepare the Brisket:

1. Trim excess fat from the brisket and apply a thin layer of mustard as a binder. Generously season with the pepper-forward rub. Let the seasoned brisket rest in the refrigerator for 1–2 hours.
2. Preheat your pellet smoker to 210°F (99°C) using hickory pellets. Place a water pan beneath the grill grates to maintain moisture.
3. Place the brisket on the smoker and cook overnight for 9–10 hours, or until it reaches an internal temperature of 165–170°F (74–77°C).
4. Remove the brisket and wrap it in butcher’s paper, spritzing the paper with apple cider vinegar and spreading beef tallow over the brisket before sealing. Increase the smoker temperature to 250°F (121°C) and continue cooking until the brisket is probe-tender or reaches an internal temperature of 204–206°F (96–97°C).
5. Keep the wrapped brisket in a cooler to rest for 2–3 hours.

Caramelize the Onions:

6. In a pan over medium heat, combine diced onions, avocado oil, kosher salt, and unsalted butter. Sauté until the onions are golden brown and caramelized. Set aside.

Assemble the Sandwiches:

7. Slice the rested brisket thinly.
8. On a slice of sourdough bread, layer fried crispy onions, a slice of Colby Jack cheese, several slices of brisket, caramelized onions, a drizzle of BBQ sauce, and a slice of Provolone cheese. Top with another slice of sourdough bread.

Toast the Sandwiches:

9. Heat a cast-iron skillet over medium heat and melt a small amount of butter. Place the assembled sandwich in the skillet and toast for 3–4 minutes on each side, or until the bread is golden brown and the cheese has melted.

Serve:

10. Remove from the skillet, slice in half if desired, and serve warm.