In a large pot, combine the water, kosher salt, granulated sugar, brown sugar, Prague Powder #1, pickling spice, mustard seeds, and garlic. Bring to a simmer over medium-high heat, stirring occasionally, until the salt and sugar have dissolved. Pour the brine into a large roasting pan and let cool, then refrigerate until chilled.

Add the brisket to the roasting pan with the brine and cover the pan with plastic wrap. Weigh the brisket down with heavy plates to keep it submerged in the brine and refrigerate 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 wire rack set over a rimmed baking sheet. Refrigerate uncovered for 24 hours.

Remove the brisket from the refrigerator and let come to room temperature.

When ready to cook, set the Traeger temperature to 225°F and preheat with the lid closed for 15 minutes.

In a spice grinder, coarsely grind the peppercorns and coriander seeds. Pat the spices all over the brisket.

Using a wired leave-in meat thermometer or a wireless one like MEATER, insert it into the middle of the thickest part of the brisket. Place the the brisket directly on the grill grates. Close the lid and smoke until the internal temperature reaches 160°F, 3 to 4 hours.

Remove the brisket from the grill and tightly wrap in foil. Return it to the grill and reinsert the thermometer if necessary. Close the lid and cook until the internal temperature reaches 204°F, 3 to 4 hours more.

Remove the brisket from the grill, partially open the wrapping, and let sit for 10 minutes to allow the steam to escape. Close up the foil and wrap the brisket in plastic wrap. Transfer to a cooler or other container and let rest for 1 hour before serving. If you prefer cold pastrami, let it cool completely, then refrigerate until chilled before slicing. Enjoy!