Remove the giblets and neck and pat the turkey dry. Place it breast-side down on a cutting board. Using sturdy poultry shears, cut along both sides of the backbone and remove it.
Flip the turkey breast-side up and press down firmly on the breastbone until it cracks and the turkey lies flat. Tuck the wing tips behind the breast.
Season all over with the salt and pepper. If you have time, refrigerate uncovered overnight for crisper skin.
Preheat the oven to 425°F. Mix the butter, garlic, thyme, rosemary, sage and paprika. Loosen the skin over the breast and thighs and rub half the butter underneath, then rub the rest over the skin.
Spread the onions and celery on a large rimmed sheet pan and pour in the broth. Set a wire rack on top (or lay the turkey directly on the vegetables) and place the turkey on it, skin-side up.
Roast for 1¼ to 1½ hours, until an instant-read thermometer reads 165°F in the thickest part of the breast and the thigh. Tent loosely with foil if the skin browns too quickly.
Rest for 20–30 minutes before carving. Use the pan drippings for gravy.