Pat turkey dry with paper towels. Place on large rimmed baking sheet or roasting pan. Mix salt and pepper; season inside and outside of turkey. Loosely cover with plastic wrap and refrigerate for 24 hr.
Preheat oven to 200°C (400°F). Again, pat turkey dry with paper towels. Make the garlic herb butter by mixing together the butter, chopped sage, chopped rosemary, garlic and paprika. Loosen the skin from the turkey breast with your fingers; rub the garlic herb butter onto the breast meat under the skin. Tuck the remaining onion, sage sprigs and rosemary sprigs inside the turkey. Tie legs together with butcher's twine and tuck wings under to prevent tips from scorching.
In bottom of clean roasting pan, place the 3 onion halves, celery, carrots and garlic cloves. Set turkey, breast-side up, on top of the vegetables. Roast in lower third of oven for 30 min. Reduce temperature to 160°C (325°F) and roast 3 hr. Cook until meat thermometer inserted into thickest part of inner thigh registers 82°C (180°F). Transfer turkey to serving platter and tent with foil for 20 min. before removing twine and slicing.
Meanwhile, strain vegetables and skim fat from pan drippings. Put drippings back into roasting pan. Place on stovetop over medium heat. Whisk 1/2 cup (125 mL) of the chicken broth with flour until smooth. Whisk mixture into the drippings. Stir in remaining chicken broth, scraping bottom of pan to incorporate into sauce. Simmer 10 to 12 min,. or until thickened. Stir in parsley, if using. Serve pan gravy with the turkey.