1. Roast turkey: Preheat oven to 425°F.
2. Rinse turkey inside and out and pat dry. Sprinkle turkey inside and out with salt and pepper, then fold neck skin under body and secure with small skewer. Stuff large cavity with oranges, 1 onion, and bay leaves. Tie drumsticks together with kitchen string and secure wings to body with small skewers.
3. Put turkey on a rack set in a large flameproof roasting pan and roast in middle of oven 30 minutes.
4. While turkey is roasting, toss remaining 2 onions with 2 tablespoons melted butter.
5. Reduce oven temperature to 350°F. Brush remaining 1/4 cup butter over turkey and roast 30 minutes more.
6. Baste turkey and scatter buttered onion wedges around it, then roast, basting turkey every 30 minutes (add a little water to pan if onions get too dark) until an instant-read thermometer inserted into fleshy part of a thigh (do not touch bone) registers 155°F, 1 1/2 to 2 hours more (total roasting time: 2 1/2 to 3 hours).
7. Transfer turkey to a platter (do not clean roasting pan) and let stand 25 minutes (temperature will rise to 165°F).
8. Make gravy: Transfer pan juices with onions to a 2-quart glass measure, then skim off and reserve 1/4 cup fat. Add enough turkey stock to pan juices to make 4 1/2 cups total.
9. Set roasting pan across 2 burners, then add 1 cup stock mixture and deglaze pan by boiling over moderately high heat, stirring and scraping up brown bits. Add remaining stock mixture and bring to a simmer.
10. Pour stock through a fine sieve back into glass measure and discard onions.
11. Whisk together reserved fat and flour in a large heavy saucepan and cook roux over moderately low heat, whisking, 3 minutes. Add hot stock mixture in a fast stream, whisking constantly to prevent lumps, then simmer, whisking occasionally, until thickened, about 10 minutes.
12. Stir in any additional turkey juices accumulated on platter and season gravy with salt and pepper.
13. Serve turkey with gravy on the side.

Serves 8-10.