Dry-brine the turkey by patting the spatchcocked bird down with paper towels on both sides until it's dry.
Carefully loosen the skin from the breast of the turkey, going through the neck, until you can slide your hand between the skin and the breast meat.
Season each turkey breast with 1 teaspoon of the kosher coarse salt, spreading it over the entire breast.
Sprinkle the remaining salt evenly over every surface of the turkey.
Place the turkey skin side up on a rimmed baking sheet, and refrigerate for 12-24 hours.
Make the herb mayonnaise while the turkey is dry-brining by placing the mayonnaise, parsley, sage, thyme, green onions, lemon zest, sea salt, and freshly cracked pepper in a tall container. Blend with an immersion blender until relatively smooth and pale green. Seal with a lid and refrigerate until needed.
Remove the turkey from the refrigerator 30 minutes before roasting to let it rest while the oven preheats.
Adjust the oven rack to lower-middle, then preheat the oven to 425 degrees.
Scatter the veggies–chopped celery, onion, and carrots on a rimmed baking sheet, then place a wire cooling rack directly on top of the veggies.
Place the dry-brined turkey on top of the wire rack, skin side down.
Prepare the turkey by slathering ½ cup of the herb mayonnaise mixture with your hands over the exposed side of the turkey, lightly coating every surface.
Flip the turkey over, skin side up, spread the legs out to the sides, and tuck the wing tips under the breast.
With your hands, spread a couple of tablespoons of the herb mayo between the skin and meat of the breast. Slather the rest of the turkey generously with the herb mayonnaise, getting it into every nook and cranny.
Roast the turkey by placing it into the oven to roast until the breast meat registers 150 degrees on a meat thermometer at the deepest point of the breast next to the breastbone, and thigh bone registers at least 165 degrees at the center joint between the drumstick and thigh or thigh and hip, 80 to 90 minutes, rotating halfway through cooking. Side Note: If any skin starts to darken too much, tent the turkey loosely with foil.
Remove the baking sheet from the oven and transfer the turkey to a cutting board to rest for 20 minutes before carving.
Make the gravy while the turkey roasts by heating the oil in a large saucepan over high heat. Add the celery, onion, carrot, garlic, chopped turkey parts, and cook, stirring occasionally, until lightly browned, about 10-15 minutes.
Add the stock, herb stems, and bay leaves. Bring to a simmer and cook gently until reduced by about half, or until the turkey is done roasting.
Strain the stock mixture through a fine mesh strainer into a large bowl, once the turkey has finished roasting and is resting.
Tilt, strain, and collect liquids from the baking sheet that the turkey was roasted on, into the same bowl, discarding the veggies from the baking sheet.
Skim off and discard most of the excess fat from the liquid.
Finish the gravy by heating the flour and butter in a medium saucepan over medium heat, stirring occasionally with a whisk until the mixture is golden brown, about 2-3 minutes.
Ladle the stock mixture into the saucepan,½ cup at a time, whisking until smooth after each addition. Once the stock is added, bring the gravy to a simmer until it reaches the desired consistency.
Stir in the soy sauce and season to taste with sea salt and freshly cracked pepper.