Preheat oven: Preheat to 325°F (165°C). Position oven rack in the lower third.
Prepare herb butter: In a bowl, combine softened butter, garlic, herbs, lemon zest, lemon juice, salt, and pepper. Mix until smooth.
Season turkey: Pat turkey dry with paper towels. Season cavity generously with salt and pepper. Stuff cavity with onion, lemon, garlic, and herb sprigs.
Apply herb butter: Gently loosen the skin over the breasts with your fingers. Spread about half the herb butter under the skin, directly on the meat. Spread remaining butter over the outside of the turkey.
Roast turkey: Place turkey breast-side up on a rack in a roasting pan. Tie legs together with kitchen twine and tuck wing tips under. Roast uncovered until internal temperature reaches 165°F (74°C) in the thickest part of the thigh (about 3–3.5 hours for a 12–14 lb turkey). Baste every 45 minutes with pan juices.
Rest turkey: Transfer to a cutting board, tent loosely with foil, and let rest for 30 minutes before carving.
Make gravy: Skim fat from pan drippings, reserving about ½ cup. Place drippings in a saucepan over medium heat. Whisk in flour to form a roux. Cook 1–2 minutes. Gradually whisk in stock until smooth and thickened. Season with salt and pepper.