The day before, combine the salt, brown sugar, juniper, peppercorns, bay leaves and lemon zest with 4 cups (1 quart) water in a medium saucepan. Bring to a boil, then turn off the heat and let the mixture sit for 30 minutes. Pour the brine into a plastic container or pot large enough to hold the turkey, add 20 cups (5 quarts) ice water and stir to combine. Submerge the turkey, adding more ice water if needed. Weigh it down with a plate to keep it submerged and refrigerate at least 8 hours and up to 12 hours.
The next day, drain, rinse and pat the turkey dry. Put on a large paper-towel-lined cutting board and let sit out at room temperature until it reaches room temperature, about 1 hour.
For the porchetta spiced butter:
Mix the butter, garlic, rosemary, fennel, lemon zest and 1/2 teaspoon pepper in a medium bowl until well combined.
Position a rack in the middle of the oven and preheat the oven to 375 degrees F.
Pat the turkey skin dry again and rub the porchetta spiced butter evenly over the surface, also getting underneath the skin of the breasts. Lay the turkey in a large roasting pan as flat as possible. If the pan is a bit small, tuck the wings behind the back with the drumsticks resting on the ledge of the pan and place a large rimmed baking sheet lined with aluminum foil at the bottom of the oven to catch drippings.
Roast until a thermometer inserted in the thigh registers 165 degrees F, 1 hour 30 minutes to 2 hours. Transfer to a cutting board and let rest 30 minutes before serving. Garnish the serving platter with lemons, rosemary and bay leaves if using.
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