1. In a large pot, combine water, kosher salt, brown sugar, peppercorns, mustard seeds, garlic, and onion. Bring to a boil, then remove from heat and let cool completely.
2. Submerge the turkey in the brine, cover, and refrigerate for 12-24 hours.
3. Preheat your smoker to 225°F (107°C).
4. Remove the turkey from the brine, rinse it under cold water, and pat it dry with paper towels.
5. Place the turkey in the smoker and cook for 4-5 hours, or until the internal temperature reaches 165°F (74°C).
6. Let the turkey rest for 20 minutes before carving.