Step 1: Prepare the Brisket - Trim excess fat from the brisket, apply mustard, coat evenly with spice mix, and marinate for 30 minutes.

Step 2: Smoke the Brisket - Preheat the smoker to 225°F, smoke the brisket for approximately 12 hours until the internal temperature reaches 195°F, then let rest for 30 minutes.

Step 3: Caramelize Onions - Warm a skillet over medium heat, add olive oil, slice and cook onions for 15-20 minutes until golden brown.

Step 4: Assemble the Sandwiches - Slice the brisket against the grain, layer on sourdough with brisket, caramelized onions, and cheese.

Step 5: Toast the Sandwich - Heat skillet with butter, place assembled sandwiches, cook 3-4 minutes per side until golden brown and cheese is melted.

Step 6: Serve and Enjoy - Let sandwiches cool for a minute, slice in half, and serve hot.