With the turkey breast-side down, use poultry shears to cut along both sides of the backbone. Open up the turkey and use the tip of a knife to score alongside the keel bone (the dark oblong bone in the middle of the breast). This makes it easier to flatten the bird. Turn the turkey breast-side up and press down strongly with the heel of your hand on the center of the breast. You should hear a crack and feel the bones give way. Allow your turkey to air-dry at room temperature for 2 hours, skin-side up.
Preheat the oven to 500 degrees F.
Rub the turkey with oil, then sprinkle all over with salt.
Put the carrots, onion and celery in the bottom of a roasting pan, tossing to combine. Create a level bed of the vegetables and place a rack on top. Place the turkey, breast-side up, on the rack. Bake for 10 minutes to sear, then turn the oven down to 350 degrees F and cook for 20 minutes per pound (5 hours to 6 hours 40 minutes). 20 minutes before complete, brush the turkey skin with the chicken reduction. Return to the oven and finish cooking.
Allow the turkey to rest for 45 minutes to 1 hour before slicing and serving.