1. In a large saucepan bring 1 quart of water and salt to a boil, stirring to dissolve salt. Remove pan from heat and stir in black peppercorns, thyme, sage, rosemary, and bay leaves. Cover and steep 5 minutes. Add 1 quart of cold water and allow to cool 5 minutes more.
2. Line a large container with a turkey brining bag. Place the turkey inside the bag. Add cold buttermilk, the last quart of cold water, and the salty herbal infusion. Secure the top of the bag and squish to combine the brine together and coat the turkey. Refrigerate for at least 24 hours or up to 48 hours.
4. Remove the turkey from the brine and discard the used brine.Pat the turkey dry, then place breast side up in a large roasting pan lined with a rack. Quarter onion and cut garlic in half. Place both inside the turkey cavity.
5. Place the turkey in the oven, with the breast centered and the legs pointing towards the back right of the oven. Roast 30 minutes, then with the breast centered, rotate the legs to the back left of the oven. Reduce the oven to 350ºF.
6. Continue to rotate the legs from left to right every 30 minutes until a meat thermometer in thickest part of the thigh reads 180º, the breast meat reads 165º, and the skin is crispy and golden brown.