Remove the turkey breast from the roasting pan; set aside.
Place the pan and its drippings over medium-low heat. Stir in the sherry, scraping and deglazing the pan. Add the mushroom pieces, continuing to stir.
As the sauce thickens, add the water, a little at a time, continuing to stir for 3 or 4 minutes. Stir in the cornstarch-water mixture, increase the heat to medium and stir until the gravy thickens.
Taste and adjust seasoning with salt and pepper, if necessary.
Servings: 3 cups