Use day-old spaghetti or chill freshly cooked noodles to prevent the casserole from becoming mushy. For extra flavor, sauté the onions and garlic in a bit of butter alongside the olive oil. Don’t skip the eggs in the cheese mixture—they help hold the casserole together beautifully. For a crispier top, broil the casserole for 2-3 minutes at the end of baking, watching closely to avoid burning. Make sure to drain the ground beef well to keep the casserole from becoming greasy. Feel free to use part-skim mozzarella for a lighter version without sacrificing meltiness.