Preheat the oven to 350 degrees F. Place the turkey in a roasting pan. Brush with the melted butter; sprinkle with the sage, salt and pepper. Roast the turkey 75 minutes, or until the centre of the roast reaches 170 degrees F on an instant read meat thermometer. Remove the turkey from the pan and set on a plate; cover with foil and rest 10 minutes. Meanwhile, drain the excess fat from the roasting pan before placing it on the stovetop over medium-high heat. Add the stock/flour mixture and bring to a simmer. Simmer until a lightly thickened gravy forms, about 4 to 5 minutes, then season with salt and pepper. Slice the turkey and serve the gravy alongside for spooning on top.