Mix the fresh hot mashed potatoes (made with potato flakes if you want) with the yolks of the eggs, the onion, herbs, and salt.
Fold in the egg whites, beaten until stiff but not dry.
Arrange half of the potatoes in a buttered 2-quart casserole.
Cover with the turkey mixed with the gravy, and top with the rest of the potatoes.
Drizzle the butter over the top and bake 35-40 minutes in a slow oven, 325°, or until well browned.