In a medium pot, bring 3 quarts of the tap water to a boil over medium heat. Put the kosher salt in a large bowl and slowly (and carefully!) pour the boiling water over the salt. Stir to blend.
Add the molasses, honey, soy sauce, red pepper flakes, sage, thyme and garlic to the salt and water mixture. Stir to blend. Add the remaining 3 quarts of cool water. Add the ice to a cooler or bucket large enough to hold the brine and the turkey. Pour the brine over the ice and use a large whisk to blend all of the ingredients.
Submerge the turkey, breast side down, in the brine. Make sure the cavity of the bird fills with the liquid as you are submerging it. Cover the cooler and allow the bird to sit in the brine overnight or for about 12 hours.
Remove the bird from the brine and dry it thoroughly with thick (absorbent) kitchen towels. Take care to wipe inside the cavity as well. Discard the brine. Whisk together the butter and the lemon zest. Gently lift the skin covering 1 breast of the turkey and spread half of the butter right on the meat under the skin. Repeat with the other breast. The butter will add extra moisture and richness as the bird roasts.
Preheat the oven to 350°F.
Arrange the turkey in a roasting pan fitted with a rack. Put on the lower rack of the oven and roast until the internal temperature of the turkey, taken from the thickest part of the thigh reads 170°F on an instant-red thermometer, about 3 1/2 hours. Remove the turkey from the oven to a cutting board or serving platter and tent with foil. Allow to rest for 15 minutes before carving and serving.