PREHEAT oven to 400°F. Line a 9"x13" rimmed baking sheet with aluminum foil and spray with nonstick cooking spray. Spread sausage in an even layer on baking sheet and place in oven while preheating.
WHISK together eggs, milk, butter, hot sauce, salt and pepper in a large bowl until well combined. Remove sheet pan from oven and scatter cheese, spinach and tomatoes over sausage. Pour egg mixture over sausage.
BAKE eggs for 15-17 minutes, until eggs are set.
CUT eggs to the size of the sandwich bread. Spread mayonnaise or mustard on bread and stack with sheet pan eggs. Eggs can be made up to 5 days ahead and stored in refrigerator for easy breakfast assembly. Eggs are also freezer-friendly: Slice eggs to size and wrap each piece individually and store in freezer-safe container until ready to eat.