Position oven rack in lowest third of oven. Heat oven to 350 degrees F. Remove neck and giblets. Rub turkey with vegetable oil.
Spread onions in a large roasting pan. Place turkey on top.
Roast in 350 degree F oven 30 minutes. Add broth. Tent with foil if browning too quickly. Roast 3 to 3-1/2 hours, until temperature registers a minimum of 180 degrees F in innermost part of thigh and 170 degrees F in breast. Remove to platter.
Scrape pan drippings into bowl. Skim fat. Melt butter in saucepan. Stir in flour. Whisk in strained pan juices and chicken broth. Add white wine and salt; simmer, stirring, until thickened, 2 to 3 minutes.