Bring a large pot of lightly salted water to boiling. Add broccoli and cook 3 minutes. Drain and set aside.
Coat a large nonstick skillet with nonstick cooking spray. Heat over medium-high heat and crumble in sausage. Cook 5 minutes, breaking apart with a spoon, until cooked through. Add tomatoes and scallions; cook 3 more minutes. Remove from heat.
Place bread cubes in a large bowl. In a medium-size bowl, whisk eggs, egg whites, milk and salt. Pour sausage mixture and broccoli florets over bread. Stir to combine. Add egg mixture and 1 1/2 cups of the cheese and stir to blend evenly. Coat a large rectangular or oval dish with nonstick spray. Transfer mixture to prepared dish, cover with plastic and refrigerate overnight.
Heat oven to 350 degrees F. Uncover dish and top with remaining 1 cup cheese. Cover with nonstick foil and bake at 350 degrees F for 45 minutes. Uncover and cook 15 minute more. Increase oven temp to broil and crisp under broiler for 3 minutes. Let rest 5 minutes before serving.