Heat oven to 375°F. Working on baking sheet, remove giblets and neck of turkey from cavities. Reserve neck and discard giblets. Pat turkey dry.  Stuff half of onions and thyme into main cavity. Tie turkey legs together with kitchen twine. Tuck wing tips underneath body (this will prevent them from burning). Rub turkey with butter (or oil) and season with 1/2 teaspoon each salt and pepper. Place turkey neck, carrots, celery, and remaining onions and thyme in large roasting pan. Place roasting rack in pan and place turkey on top. Roast turkey until thermometer inserted into thickest part of thigh registers 165°F, 2 1/2 to 3 hours. (Cover turkey loosely with foil if it browns too quickly, and add broth to pan if vegetables begin to scorch.) Carefully tilt turkey to empty juices from cavity into pan. Transfer turkey to carving board or cutting board set in rimmed baking sheet (to catch juices while carving). Cover loosely with foil and let rest at least 30 minutes. Reserve pan and contents to make gravy. Carve and garnish turkey as desired.