Preheat the oven to 350 degrees.
Spray a 9x13 casserole dish with nonstick cooking spray.
In a large mixing bowl, whisk together the eggs and milk until completely mixed.
¼ cup milk, 4 eggs
Stir in the grated Parmesan cheese.
½ cup grated Parmesan cheese
Add the spaghetti to the bowl and toss the egg mixture until well combined.
8 cups leftover spaghetti with sauce
Add 1 cup of the cheddar cheese and toss until mixed.
2 cups shredded sharp cheddar cheese OR mozzarella cheese
Transfer the spaghetti to the prepared baking dish.
Bake, covered, for 30 to 40 minutes or until the casserole is cooked through.
Remove the cover and top the casserole with the remaining cheese.
Return to the oven and bake for an additional 5 to 10 minutes or until the cheese has melted.