1. Preheat oven to 375 degrees F. Spray a large baking dish and a large skillet with vegetable spray. Sauté mushrooms and onions over medium heat until softened.
2. Combine cottage cheese and spinach. Spoon half of tomato sauce into prepared dish. Arrange half of bread over sauce. Spoon cheese mixture over bread. Top with remaining bread, onion mixture and mozzarella. Spoon remaining sauce on top.
3. Whisk together milk, eggs and egg whites. Pour mixture over strata. Pierce the bread with the tip of a knife until liquid is absorbed. Chill 30 minutes. Sprinkle with Parmesan. Bake 50 minutes.