In a large pot, combine water with the kosher salt, granulated and brown sugars, pink salt, pickling spice, mustard seeds and garlic. Cook over moderately high heat, stirring occasionally, until the salts and sugars have dissolved. Pour the brine into a large bowl and let cool. Refrigerate until chilled.
Using a metal skewer, poke holes all over the brisket. Pour the brine into a large roasting pan. Add the brisket and cover the pan with plastic wrap. Weigh the brisket down with heavy plates to keep it submerged in the brine and refrigerate. Let the brisket cure for 5 days, turning it once a day.
Remove the brisket from the brine and pat dry with paper towels. Place the brisket fat side up on a rack set on a rimmed baking sheet. Refrigerate, uncovered, for 24 hours.
Remove from fridge and bring the brisket to room temperature.
When ready to cook, start your Traeger grill on Smoke with lid open until fire is established (4-5 minutes). Increase temperature to 225 degrees F and preheat with lid closed, 10 to 15 minutes.
In a spice grinder, coarsely grind the peppercorns with the coriander seeds. Cover the fat side of the brisket with the peppercorn-coriander mixture.
Place the brisket, spiced side up, in the center. Close lid and smoke at 225 degrees F until an instant-read thermometer inserted in the thickest part registers an internal temperature of 160 degrees F (about 3 to 4 hours).
Remove from grill and wrap in foil making sure to seal very well. Place back on grill and continue cooking for another 3 to 4 hours or until a thermometer inserted in the thickest part of the meat registers an internal temperature of 204 degrees F.
Remove from grill and rest in foil for at least 30 minutes before slicing. If you want cold pastrami, allow to cool completely, refrigerate until chilled and then slice. Enjoy!