Fry sausage until browned and almost cooked through. Add scallion bottoms along with 1/3 of the green tops and cook until meat is thoroughly cooked and turn off heat. Add chilies and mix well.
Spray a 9X13 inch-baking dish with cooking spray. Place the bread slices into the dish to cover bottom and pour sausage mixture evenly over bread. Spread cheese evenly over sausage and sprinkle reserved diced green scallion tops and tomato over cheese.
In a separate bowl, beat eggs and add half-and-half and pour over casserole. Cover with plastic wrap and refrigerate overnight.
Preheat oven to 350 degrees and bake casserole for 35 minutes. Cover with aluminum foil and continue to bake another 15 minutes. Remove from oven and let stand for 5 minutes before serving.
Prep time does not include overnight resting period.