Preheat the oven to 350 degrees F. Set the turkey in a small roasting pan; season with salt and pepper. Roast 60 minutes, or until cooked through. Transfer to plate, cover with foil and let rest 10 minutes. Drain all but 1 Tbsp. of the excess fat from the roasting pan and set on the stovetop over medium-high heat. Add the onion and cook 3 to 4 minutes. Whisk the flour and stock together in a bowl until lump-free, and then pour into the roasting pan. Mix in the sage; simmer 5 minutes and then season with salt and pepper. Slice the turkey and serve the gravy alongside for spooning on top.