Spray a 9-by-13-inch baking dish with cooking spray.
Whisk together the milk, salt, cayenne, eggs, egg whites and scallions in a large bowl. Stir in the bread cubes and sausage and let stand 15 minutes, stirring occasionally.
Spoon the bread mixture into the baking dish. Cover and refrigerate at least 8 hours or overnight.
Preheat the oven to 400 degrees F.
Remove the casserole from the refrigerator and let stand 10 minutes. Evenly sprinkle the cheese over the top and bake uncovered until puffed and golden, and a knife inserted in the center comes out clean, about 45 minutes. Let stand 5 minutes before serving.
Per Serving: 192 Cal; 16 g Protein; 6 g Tot Fat; 2 g Sat Fat; 2 g Mono Fat; 19 g Carb; 2 g Fiber; 6 g Sugar; 249 mg Calcium; 2 mg Iron; 553 mg Sodium; 82 mg Cholesterol