Lightly mist a round 2-quart casserole dish with cooking spray. Alternately place the ham and muffins in the dish to make a shingled pattern. Sprinkle the sundried tomatoes, scallions and 1/3 cup cheddar over the casserole.
In a large bowl, whisk the eggs, egg whites, milk, and mustard; season with salt and pepper. Pour the custard filling over the casserole and sprinkle with the remaining 1/3 cup cheese. Cover the casserole with plastic wrap; refrigerate at least 5 hours and up to overnight.
Preheat the oven to 350 degrees F. Place the casserole on a large rimmed baking sheet; bake until golden around edges and just set, about 1 hour. Let stand 10 minutes before serving.
Delicious, comfort food for an easy brunch on Sunday. We cut the fat using reduced fat milk instead of half and half or cream. But you'll hardly miss it.