When ready to cook, start your Traeger on Smoke with the lid open until the fire is established (4-5 minutes). Set the temperature to 225F and preheat, lid closed, (10-15 minutes).
Combine rub ingredients in a small bowl and stir to mix. Season the brisket generously on both sides.
Put a wire cooling rack in a rimmed baking sheet or other shallow pan and place the brisket, fat-side up, on the cooling rack. Place pan and brisket on the grill grate.
Combine all the ingredients for the mop sauce in a nonreactive saucepan over medium heat. Bring to a simmer and stir until the salt dissolves. Re-warm before mopping the brisket so the fat (bacon grease, lard, or butter) liquefies.
After the first hour, mop the brisket with the mop sauce; continue to mop every hour for the first 4 hours.
When the internal temperature of the meat reaches 165F, wrap the meat tightly in butcher paper. (Note: Butcher paper is more permeable, but if you don't have access to any, use foil.) Return the meat to the grill and continue to cook until the internal temperature reaches 203F. (Total cooking time will vary, but plan on 6 to 8 hours total.)
Transfer the wrapped meat to an insulated cooler thickly lined with newspapers or bath towels. Let the meat rest for at least 30 minutes. Reserve any juices that have accumulated in the baking sheet.
Unwrap the brisket. Thinly slice across the grain using an electric knife or sharp carving knife. Shingle the slices of brisket on a platter and pour the pan juices on top.