In a small bowl, dissolve the cornstarch in the water. Set aside.
In a skillet over medium-high heat, brown the onion and garlic in the oil. Add the broth, sherry and reserved drippings from the Turkey Roast, if desired. Bring to a boil and simmer over high heat for about 15 minutes or until the gravy reduces by half.
Drizzle in the cornstarch mixture, whisking continuously. Simmer for 1 minute or until the gravy thickens. Season with salt and pepper.