Preheat the oven to 350 degrees F. Place the turkey in a roasting pan. Brush with the melted butter and season with salt and pepper. Roast the turkey for 70 to 80 minutes, or until the internal temperature of the breast reaches 170 degrees F on a meat thermometer.
When the turkey is almost cooked, heat the oil in a pot over medium heat. Add the mushrooms and garlic and cook until tender, about 5 minutes. Mix in the flour until well combined and cook 2 minutes more. Slowly mix in the stock. Stir in the sage. Bring the sauce to a simmer and simmer 5 minutes. Stir in the whipping cream and season with salt and pepper. Cover the sauce and reserve on low heat.
When the turkey is cooked, transfer to a platter and let it rest for 10 minutes before thinly slicing and arranging on the platter. Stir any pan juices into the sauce. Serve the sauce with the turkey.