Wash the broccoli well. Cut into small florets. Boil water in a pot. Cook broccoli for 3 minutes only. Drain and set aside. Do not overcook. You want it slightly firm.

Set your oven to 180°C (350°F). This ensures even cooking. While it heats, you can prepare the mixture. Grease your baking dish lightly to prevent sticking.

In a large bowl, add mayonnaise and cream of mushroom soup. Mix until smooth. Add eggs and whisk well. This creates a creamy and rich base.

Now add chopped onion, salt, and pepper. Stir everything together. The onion adds a slight crunch and strong flavor. Mix until evenly combined.

Put the cooked broccoli into the bowl. Add cheddar and mozzarella cheese. Gently fold everything together. Make sure broccoli is fully coated.

Pour the mixture into your greased baking dish. Spread it evenly using a spoon. This helps it cook properly and look neat when served.

In a small bowl, mix crushed crackers with melted butter. Stir well. This topping will become crispy and golden in the oven.

Sprinkle the cracker mixture over the casserole. Cover the entire surface. This layer adds a perfect crunch to your dish.

Place the dish in the oven. Bake for 30 minutes. The top should turn golden brown. The inside will be creamy and hot.

Take the casserole out carefully. Let it rest for 5 minutes. This helps it set. Serve warm and enjoy every bite.