Place the turkey on a cutting board with the backbone facing up. Using kitchen shears, cut along both sides of the backbone to release it from the bird. Remove the backbone and discard. Flip the turkey over and use your palms to press against the turkey breasts and thighs until the bird is slightly flattened. Allow the turkey to come up to temperature for 2 hours.
Preheat the oven to 425 degrees F.
Remove 2 tablespoons of ranch seasoning from 1 packet and reserve. Mix together 8 tablespoons of the butter with the remaining ranch seasoning (from both packets) in a small bowl until well incorporated.
Line a rimmed baking sheet with aluminum foil. Place the carrots, celery and onions over the baking sheet. Lay the turkey on top of the vegetables, breast-side up. Spread the ranch butter all over and underneath the skin. Sprinkle generously with salt and pepper. Reduce the oven temperature to 350 degrees F and put the turkey inside to roast.
In the meantime, add the maple syrup and 1 cup of the chicken stock to a saucepan, then stir in the reserved 2 tablespoons ranch seasoning. Bring to a simmer just to warm through and incorporate the seasoning. Baste the turkey with this liquid every 20 minutes. (Allow 30 to 40 minutes of cooking time for every 3 pounds of turkey). If the skin turns golden brown before the turkey is completely cooked through, tent with aluminum foil until an instant-read thermometer inserted into the thickest part of the thigh reads 160 degrees F. Remove from the oven and tent the whole turkey with aluminum foil. Let rest 30 minutes. (There will be carryover cooking and the temperature will rise to 165 degrees F while resting).
For the gravy, strain the juices left in the bottom of the roasting pan into a fat separator and pour the fat off the top. Place the saucepan used to make the basting liquid back on the stove over medium heat. Add the remaining 2 tablespoons butter and whisk in the flour, cooking for a minute. Slowly whisk in the remaining 2 cups chicken stock along with the juices from the pan. Season with salt and pepper and serve alongside the turkey.