Preheat the oven to 400 degrees F. Place the mirepoix in a deep roasting pan . Lay the turkey on the mirepoix. Roast for 15 to 20 minutes to get the browning process going. Reduce the heat to 350 degrees F. Cover with a lid and bake for 4 to 4-1/2 hours, or until juices run clear. Bast the turkey every 30 minutes with the melted butter. Remove from the oven and let cool for 10 minutes. Lift the turkey out of the pan and carve. Serve warm with pan juices.