Heat a large nonstick skillet over medium heat. Add the turkey and scallions and cook, stirring to break up any large chunks, until browned and cooked through, about 10 minutes. Remove from heat and let cool slightly.
Whisk the eggs, egg whites, milk and 1/2 teaspoon each salt and pepper in a large bowl until combined. Add the cooked sausage, spinach, cheeses and bread and toss to distribute ingredients evenly.
Spray a 3-quart casserole dish with cooking spray. Spread the egg mixture evenly in the dish. Cover and refrigerate for at least 6 hours or preferably overnight.
Preheat the oven to 175°C. Bake the casserole, uncovered, until set and lightly browned on top, about 30 minutes.
Make-Ahead Tip: Freeze the baked casserole for up to 2 weeks. Cover with foil and reheat at 175°C until hot, 35 to 45 minutes.