Preheat the oven to 350 degrees F. Dust the inside of a large (14-by-20-inch) oven bag with flour. Rinse turkey under cold running water; pat dry with paper towels. Season the breast all over with salt and pepper. Rub butter over the skin side, then place the breast skin side down. Place onion quarters and bay leaves in the breast cavity. Place the turkey breast in the oven bag. Gather the bag loosely around the turkey, allowing room for heat to circulate. Secure the bag with its nylon tie. Cut six 1-inch slits in the top of the bag. Place the bag in a 13-by-9-inch (3-quart) baking dish. Bake the turkey until it is deep golden brown and the meat is cooked through, 1 1/2 to 2 hours. Let the turkey rest in the bag for 15 minutes, then transfer it to a platter to slice. Remove and discard the bay leaves and onion quarters. Reserve the pan juices. Strain them for making gravy, if desired, or freeze them for making turkey soup.