Prepare the curing brine by combining 1 gallon cold water, 1 cup kosher salt, 0.75 cup dark brown sugar, 3 tablespoons Prague Powder #1, 0.25 cup pickling spices, and 8 smashed garlic cloves in a large pot. Heat over medium heat, stirring occasionally, until salt and sugar fully dissolve (about 5 minutes). Remove from heat and cool to room temperature, then refrigerate until completely cold (below 40°F).

Place the 4 lb beef brisket flat in a large food-safe container or heavy-duty zip-top bag. Pour the chilled brine over the brisket, ensuring the meat is fully submerged. If needed, weigh down the brisket with a plate. Seal the container or bag and refrigerate for 7 days, flipping the brisket once daily to ensure even curing.

After 7 days, remove the brisket from the brine and rinse thoroughly under cold running water for 3 minutes, rubbing the surface to remove all surface cure. Pat completely dry with paper towels.

Place the rinsed brisket uncovered on a wire rack set over a sheet pan. Refrigerate uncovered for 24 hours to form a pellicle (slightly tacky surface that helps smoke and seasoning adhere).

Combine 4 tablespoons coarsely cracked black peppercorns and 4 tablespoons coarsely cracked coriander seeds in a small bowl. If using, apply a thin layer of yellow mustard to all surfaces of the brisket. Press the peppercorn-coriander mixture firmly into all surfaces of the meat, creating an even crust.

Preheat your smoker to 225°F using oak, hickory, or fruit wood. Insert a probe thermometer into the thickest part of the brisket flat.

Place the seasoned brisket directly on the smoker grate fat-side up. Smoke uncovered for 4 hours or until internal temperature reaches 165°F, maintaining steady smoker temperature at 225°F.

When brisket reaches 165°F internal temperature, remove from smoker and wrap tightly in butcher paper or heavy-duty aluminum foil. Return wrapped brisket to smoker.

Continue cooking wrapped brisket for 4 hours or until internal temperature reaches 203°F. The pastrami should feel probe-tender when tested with a thermometer or skewer.

Remove finished pastrami from smoker and rest wrapped for 60 minutes in a cooler or 170°F oven to allow juices to redistribute.

Unwrap the rested pastrami and slice against the grain to 0.125-inch thickness using a sharp knife or electric slicer. Serve immediately on rye bread with mustard and sauerkraut, or refrigerate for up to 7 days.