Coat an ovenproof nonstick skillet with cooking spray and place over medium heat. Put stuffing in the bottom of pan to warm and stir to break up a little. Sprinkle cheese over top.
In a medium bowl, whisk together eggs, egg whites, milk, mustard powder, and nutmeg. Pour mixture over stuffing and cheese to cover stuffing. Sprinkle Parmesan over top. Cook on stovetop over low heat for 5 to 7 minutes. Transfer to the broiler and broil for about 2 minutes, until egg is cooked through and cheese is golden and bubbly.
Sprinkle with salt, pepper, and fresh parsley. Serve with sliced tomatoes.