Set a rack in lower third of oven; preheat oven to 325°F. Combine oil, butter, garlic, chopped herbs and 1 tsp. each salt and pepper.
Remove neck and giblets; discard. Rinse turkey; pat dry. Rub oil-herb mixture over turkey and under skin. Sprinkle salt and pepper inside bird; then stuff it with lemon, onion and herb sprigs. Tuck wings under; tie legs with kitchen twine.
Place turkey, breast side up, on rack in roasting pan. Cover loosely with foil. Roast for 3 hours. Remove foil; roast for 1 to 1 1/2 hours, basting every 15 minutes, until a meat thermometer inserted in thickest part of thigh reaches 175°F.
Place turkey on a cutting board. Tent with foil; let rest for 30 minutes before carving.