Combine the salt, sugar, coriander, mustard seeds, peppercorns, garlic, bay leaves and ginger with 4 cups (1 quart) water in a medium saucepan. Bring to a boil and then turn off the heat and let sit 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.
Preheat the oven to 375 degrees F. Position a rack in the middle of the oven.
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.
Pat the skin dry again and sprinkle the turkey inside and out with pepper. Scatter the bottom of a large roasting pan with the whole garlic if using. Lay the turkey over the garlic 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 your oven to catch drippings. Roast until a meat thermometer inserted into the thigh registers 155 degrees F, 1 hour 30 minutes to 2 hours. Transfer to a cutting board and let rest 30 minutes before serving. Garnish with the roasted garlic and parsley sprigs if using.
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