Rinse the brisket, pat it dry and trim the fat cap, so it is 1/8 inch thick.
Bring the water to a boil in a medium pot.
Reduce to a simmer once boiling, then add the kosher salt, sugars, pink curing salt, coriander seeds, pickling spice, mustard seeds, garlic, red pepper flakes, and bay leaves.
Simmer for 12-15 minutes.
Remove the brine from the heat and set it aside for 15 minutes to cool down.
Add the cool water until the brine temperature reaches 40 degrees Fahrenheit.
Transfer the brisket to a large non-reactive container.
Pour the brine over the brisket, cover the container, and refrigerate for 5-6 days. Flip the brisket flat once every day.
Remove the brisket flat from the brine and pat dry with paper towels.
Preheat your Z-Grill (preferably with branded wood pellets) or other wood pellet grill to 250 degrees Fahrenheit.
Crush the coriander and mustard seeds with a meat mallet, mortar and pestle, or rolling pin.
Add to a bowl with the black pepper and garlic powder. Mix to make the spice rub.
Apply the vegetable oil to the brisket, then add the spice rub. Apply the rub all over the brisket flat.
Transfer the brisket to the grates, close the lid, and smoke for 5-6 hours or until the internal temperature reaches 165-170 degrees Fahrenheit.
Fill a spray bottle with beef broth and spritz the beef with beef broth every hour.
Increase the pellet grill’s temperature to 300 degrees Fahrenheit and cook for 1 hour and 30 minutes or until the internal temperature reaches 200 degrees Fahrenheit.
Remove the pastrami from the smoker and rest for 30-40 minutes.
Thinly slice the pastrami against the grain using a sharp knife or load it onto a deli slicer, then serve as desired. It tastes great sandwiched between two slices of toasted bread with mustard and sauteed onions.