Remove the wet-aged brisket flat from the packaging, pat dry with paper towels and place on the cutting board and trim off any excess fat. There isn't usually much of it on brisket flats.
Sprinkle a light coating of kosher salt on one side, then a light coating of the coarse pepper. Pat with your hand to help the seasonings stick. Flip the meat and repeat. Don't forget to season the sides as well. The amount of seasoning is up to you. Add more if you like. If you like your brisket more peppery, add more pepper.
Leave the meat rest at room temperature to get rid of some of the chill. Meanwhile, prepare the smoker.
Preheat the smoker to 275F (140C) using hickory, or oak supplemented with mesquite chips for smoke. I recommend starting smoking at 225F (107C) for 30 min, increase the temperature to 250F (121C) and smoke for another 30 minutes, then let temperature creep up to 275F and keep it there. This way the meat won't get shocked by intense heat and won't sweat.
Place the brisket fat side up and smoke until the internal temperature reaches 195F (90C).If wrapping in butcher paper (see my notes in the post above) smoke until the internal temperature reads 175F (80C) on a meat thermometer. Tear off a large sheet of butcher paper. Wrap the brisket flat as tightly as possible and place back on the rack, seam side down making sure it won't unwrap. Continue to smoke until the internal temperature reaches 195F (90C).
Rest the brisket for about 30 minutes at room temperature or hold in a faux cambro for up to 4 hours.
Slice the brisket flat against the grain and serve with white bread, sliced onions, and jalapeños.