Remove the corned beef from its packaging and rinse it under cold water to wash away excess brine and salt.
In a large mixing bowl, combine black peppercorns, red peppercorns, coriander seeds, mustard powder, smoked paprika, garlic powder, and onion powder. Optionally add brown sugar. Crush the spices for better flavor release.
Rub the spice mixture generously over the corned beef. Wrap tightly in plastic wrap and refrigerate for at least 5 days.
After curing, rinse the meat under cold water and pat it dry with paper towels.
For a smoky flavor, preheat your smoker to 225°F (107°C) and smoke the pastrami for 6-8 hours until it reaches 190°F (88°C). For oven roasting, preheat to 300°F (150°C) and roast for 3-4 hours in a pan with water.
Once cooked, let the pastrami rest for at least 30 minutes before slicing thinly against the grain.
