Plan ahead! This requires overnight time.
For the brine, bring the water to a boil in a large pot. Stir in the garlic, juniper berries, bay leaves, salt, brown sugar, curing salt, if using, whole peppercorns, and allspice berries. Let cool completely, then immerse the meat. (Weight it down with a plate if necessary.) Refrigerate for 3 days.
For the rub, in a spice grinder or small food processor, combine the black pepper, coriander seeds, mustard seeds, and bay leaves. Pulse until coarsely ground. Stir in the brown sugar, paprika, cinnamon, and clove.
Remove the meat from the brine and rinse under cold running water. Pat dry with paper towels. Sprinkle and pat the rub on the brisket. Cover tightly with plastic wrap. Refrigerate for 24 hours.
When ready to cook, start the Traeger grill on Smoke with the lid open until the fire is established (4 to 5 minutes).
Remove the plastic wrap from the beef brisket and arrange on the grill grate. Smoke the meat for 3 to 4 hours.
Wrap the brisket tightly in foil. Increase the temperature to 250, and continue to cook the brisket until the internal temperature is 175.
Allow the brisket to cool to room temperature, leaving it in the foil. Transfer the brisket to a baking dish, and weight it with foil-covered bricks or cast iron skillets. Refrigerate overnight. Slice thinly against the grain for serving. (A meat slicer is preferred to a knife.)