1. Preheat oven to 375ºF. Coat a large nonstick baking sheet with cooking spray.
2. Cut bread into cubes and spread out on sheet in a single layer. Coat with cooking spray; toast in oven until lightly browned, about 5 minutes. Remove from oven and spoon into a large bowl; set aside.
3. Heat a large nonstick skillet over medium-high heat; add oil. Saute onion until it starts to turn transparent, about 2 to 3 minutes. Add garlic and bacon; cook, stirring occasionally, until onion starts to brown, about 3 to 4 minutes more. Remove skillet from heat and set aside.
4. Place spinach, squash, eggs, egg whites, milk, salt and pepper in a large bowl; whisk to combine. Add onion mixture; whisk again. Pour over toasted bread cubes and mix with a spoon to thoroughly combine.
5. Pour mixture into a 9x13-inch nonstick pan and spread into an even layer; sprinkle with cheese. Bake until a knife inserted in center comes out clean, about 20 to 25 minutes. Slice into 8 pieces and serve.

Serving size: Yields 1 piece per serving.