Preheat oven to 400 degrees. Coat a 9 x 9-inch baking pan with cooking spray.
To Cook Bacon: Line a plate with several paper towels. Place 4 bacon strips on paper towels. Cover with several paper towels. Cook in the microwave for 4-5 minutes until well done and crispy. Repeat with 4 remaining bacon strips. Add all cooked strips to clean paper towels and blot, cool, and crumble. Set aside.
Cook onions in microwave for 2 minutes, to soften.
In a large bowl, mix together frozen diced hash browns, cooked crumbled bacon, tomatoes, and soften onions. In a medium bowl, whisk together eggs, egg whites, condensed soup, milk, black pepper, and a little salt. Pour over potato mixture and toss to mix. Add mixture to baking pan. Cover with foil and bake for 30 minutes. Remove foil, sprinkle with cheese and bake an additional 15-18 minutes until golden brown and cheese is melted.
Let sit about 10 minutes to set. Cut into 6 slices. Using a spatula, carefully remove each slice.
Once baked, this casserole freezes great. I like to wrap into individual servings and store in the freezer. Great for a quick breakfast any day of the week!