a 5-gallon bucket for brining; an instant-read thermometer
For the brine: Heat the water, thyme, garlic, onion, lemon zest, peppercorns, bay leaves, kosher salt and light brown sugar in a large saucepan over medium-high heat, stirring frequently, until the sugar and salt dissolve. Transfer the liquid to a clean 5-gallon bucket. Add enough ice to total 3 gallons of liquid and mix until incorporated.
For the turkey: Add the turkey to the bucket with the brining liquid and refrigerate for at least 12 hours.
When ready to roast, preheat the oven to 450 degrees F. Have ready a large roasting pan with a rack.
Remove the turkey from the brining liquid and use paper towels to pat it as dry as possible inside and out. Rub the turkey with the butter all over, including the cavity, and sprinkle with the sea salt and pepper. Stuff the cavity with the thyme, garlic, onions, bay leaves and lemon zest from the brine solution. Fold the wingtips underneath the turkey. Truss the legs with kitchen twine. Place the turkey in the roasting pan, breast-side up. Fold the neck flap under.
Roast the turkey for 1 hour to brown it, then reduce the oven temperature to 350 degrees F and cook for another 1 1/2 to 2 hours or until a thermometer inserted in the thickest part of the thigh registers 165 degrees F. Let the turkey rest at room temperature, covered with foil, for 30 minutes before carving.