Preheat oven to 350 degrees F (175 degrees C). Lightly grease a 9x13 inch baking dish.
Melt 3 tablespoons butter in a saucepan over low heat. Blend in the flour. Slowly stir in evaporated milk and water, then season with salt, pepper, and onion powder. Stir sauce over low heat for 5 minutes.
In a separate saucepan over low heat, melt 2 tablespoons butter. Blend in the dry stuffing mix. Place the turkey in the prepared baking dish. Pour the sauce over turkey, then sprinkle with Cheddar cheese. Spread mashed potatoes over cheese. Top mashed potatoes with the stuffing mixture.