Plan ahead! This recipe requires brining for at least 8 hours.
Thaw the turkey, if frozen, 3 days before you plan to eat the bird. Remove the giblets, if any.
For the brine, in a large 5-gallon bucket or clean cooler, combine the beer, water, salt, and brown sugar and stir with a long-handled wooden spoon until the salt and sugar crystals are dissolved. Add the garlic and peppercorns.
Add the turkey. Keep it submerged with a heavy pot lid or resealable bags of ice. Refrigerate for at least 8 hours.
Remove the bird from the brine. Dry the turkey thoroughly, inside and out, with paper towels. Discard the brine.
Put the onion, herbs, bay leaves, celery, and apple in the main cavity and tie the legs together with butcher's twine. Fold the wings behind the bird's back. Oil the outside of the bird.
Put the turkey on a rack in a sturdy roasting pan. If you don't care to save the drippings, you can put the turkey directly on the grill grate.
When ready to cook, start the Traeger grill on Smoke with the lid open until the fire is established (4 to 5 minutes). Set the temperature to 350F and preheat, lid closed, for 10 to 15 minutes.
Put the roasting pan with the turkey on the grill grate and roast for 2-1/2 to 3 hours, or until a temperature probe inserted in the thickest part of the thigh reads 165F.
Let the turkey rest for 20 minutes before carving.