Step 1: Prepare the Broccoli

• If using fresh broccoli: Bring a large pot of salted water to a rolling boil. Add the fresh broccoli florets and blanch them for 2-3 minutes. You’re not trying to cook them through, just to tenderize them slightly and bring out their vibrant green color. Immediately drain the broccoli in a colander and rinse with cold water (or plunge into an ice bath) to stop the cooking process. Drain very thoroughly. Patting the florets with a paper towel is highly recommended. This is the most crucial step to prevent a watery casserole.
• If using frozen broccoli: Thaw the broccoli completely according to package directions. Place the thawed broccoli in a colander and press down firmly to squeeze out as much excess water as possible. You’ll be surprised how much liquid comes out. Again, pat dry with paper towels.

Step 2: Prepare the Baking Dish and Oven

• Preheat your oven to 350°F (175°C).
• Grease a 9×13 inch baking dish (or a similar 2.5 to 3-quart casserole dish) with butter or non-stick cooking spray. Set aside.

Step 3: Make the Creamy Cheese Sauce

• In a large mixing bowl, combine the cream of mushroom soup, mayonnaise, beaten eggs, finely chopped onion, salt, and black pepper. Whisk until the mixture is smooth and well-incorporated.
• Stir in 1 ½ cups of the shredded cheddar cheese, reserving the remaining ½ cup for later. Mix until the cheese is evenly distributed throughout the sauce.

Step 4: Combine and Assemble the Casserole

• Add the thoroughly drained and dried broccoli florets to the bowl with the cheese sauce.
• Gently fold everything together using a rubber spatula, taking care not to break up the broccoli florets too much. Ensure every piece of broccoli is generously coated in the creamy sauce.
• Pour the broccoli mixture into your prepared baking dish and spread it into an even layer.
• Sprinkle the remaining ½ cup of shredded cheddar cheese evenly over the top of the broccoli mixture.

Step 5: Prepare the Topping and Bake

• In a small bowl, combine the crushed butter crackers with the 4 tablespoons of melted butter. Mix until all the crumbs are moistened.
• Sprinkle the buttered cracker crumbs evenly over the layer of cheese.
• Place the casserole dish on a baking sheet (to catch any potential drips) and bake in the preheated oven for 30-40 minutes.
• The casserole is done when the edges are bubbly, the center is set and heated through, and the cracker topping is a beautiful golden brown.

Step 6: Rest and Serve

• Carefully remove the casserole from the oven and let it rest on a wire rack for at least 10-15 minutes before serving. This crucial resting period allows the casserole to set, making it easier to serve and preventing the sauce from being too runny.