Crumble sausage into a medium nonstick skillet. Cook over medium heat until nicely brown, breaking up into bits with spatula as it cooks. Sprinkle with poultry seasoning. Coat a 9 x 13 inch baking dish with canola cooking spray and set aside.
Add sausage to large bowl along with toasted bread squares, cheese, mustard powder and salt, if desired. Add whole eggs, egg whites (or egg substitute) and milk to mixing bowl, and beat on medium-low speed until smooth and completely blended.
Drizzle egg-milk mixture over the sausage and bread mixture; stir to blend. Pour into the prepared baking dish, spread top evenly, cover with foil, then chill in the refrigerator for 8 hours or overnight.
Preheat oven to 350 degrees and bake, covered in foil, for 45 minutes. Uncover the foil, and reduce the temperature to 325 degrees. Bake for about 20 minutes longer or until set.