This is a vegetable dish perfect for brunch or a cool afternoon picnic. Make this up to two days ahead and reheat for those days when you just don’t feel like cooking anything.
Preheat the oven to 375 degrees.
Coat a large nonstick baking sheet with cooking spray. Spread the bread out on the sheet in a single layer. Coat with cooking spray and toast in the oven until lightly browned, about 5 minutes. Remove from oven and spoon into a large bowl. Set aside.
Heat a large nonstick skillet over medium-high heat and add the oil. Sauté the onion until it starts to turn transparent, about 2-3 minutes. Add the garlic and bacon. Cook, stirring occasionally, until the onion starts to brown, about 3-4 more minutes. Remove the skillet from heat and set aside.
Place the spinach, squash, eggs, egg whites, milk, salt, and pepper in a large bowl and whisk to combine. Add the onion mixture and whisk again. Pour the mixture over the toasted bread cubes and mix with a spoon to thoroughly combine.
Pour the mixture into a 9-by-13-inch nonstick pan and spread into an even layer. Sprinkle with cheese. Bake until a knife inserted in center comes out clean, about 20-25 minutes. Slice into 8 pieces and serve.