Fully thaw the frozen turkey. It's best done in the fridge and a few days in advance. Prepare the turkey brine, and place turkey in the brine in a food safe container. Place in fridge for up to 24 hours to brine.
Remove turkey from brine, pat dry, or place in fridge uncovered for a few hours to dry. Drizzle oil over the turkey and rub in seasoning salt all over the turkey, including inside the turkey cavity.
Stuff the onions and apples inside the turkey. Using string, tie the turkey legs together. It looks nicer when serving.
On day of smoking, light and heat your off set smoker to 275°C, according to your manufacturers instructions. Or place lit coals on the side of a bbq, away from where your turkey will sit. When charcoals are lit for at least 20 minutes, then add the apple wood chunks.
Place internal thermometer probe inside the turkey breast and into the deepest part of the thigh, without touching the bone. Place turkey in your smoker and allow to cook for 30 to 40 minutes per pound. Internal temperature of the breast must reach 165°F and thigh at 180°F in the thickest part of the meat. Use a stick thermometer to check the temperature before removing the turkey to ensure correct temperature is reached.
Wearing silicone gloves remove turkey from the smoker. Place it in a tray, cover with foil to allow to rest for 30 minutes before serving.