- Place the turkey into a pot or bucket.
- Add the apple cider, kosher salt, sugar, and enough water to just cover the turkey (should be about 1 gallon), and stir to dissolve the sugar and salt.
- Cover and refrigerate for 16-24 hours.
- After brining, rinse with water to remove excess salt and then thoroughly pat dry with paper towels.
- Place the turkey onto a roasting pan and stuff with the onion and apple quarters.
- In a small bowl, stir together the sea salt and rosemary.
- Brush the turkey all over with the melted butter and then sprinkle with the rosemary sea salt.
- Preheat the oven to 425 degrees F.
- Place the turkey in the oven on the lowest rack and cook for 30 minutes.
- Reduce the temperature to 325 and cook another 2-2 ½ hours, depending on the size of your turkey, basting the turkey with the remaining apple cider every 30-45 minutes.
- Test the internal temperature of the turkey into the thigh and the breast. The turkey is done once the temperature reaches 165 degrees F.
- Cover with aluminum foil and let rest for 20 minutes before slicing turkey.