Preheat your oven to 400°F (205°C) to ensure it reaches the correct temperature for baking. While the oven is heating, lightly grease a 13x9-inch casserole dish with cooking spray to prevent sticking. This step ensures even cooking and easy cleanup. Make sure the oven racks are positioned correctly so that the dish will be in the middle of the oven.
Fill a large bowl with ice water to prepare for blanching the broccoli. Blanching involves briefly boiling the broccoli, then immediately shocking it in cold water to stop the cooking process and preserve its color and texture. This step helps maintain the vibrant green color of the broccoli and makes it more tender-crisp.
Bring a large pot of salted water to a boil over medium-high heat. Add the broccoli and cook, stirring occasionally, until it turns bright green and becomes crisp-tender in about 2 minutes. This ensures the broccoli retains its vibrant color and texture. Use a slotted spoon to carefully transfer the cooked broccoli into a bowl of ice water.
Let it sit for 1 to 2 minutes to halt the cooking process and maintain its freshness. Drain the broccoli thoroughly to remove excess moisture, which will help in achieving a better texture when baking later.
In a large bowl, combine the broccoli, soup, mayonnaise, cheese, eggs, onion, salt, and pepper until the mixture is well blended and evenly combined. Ensure all ingredients are fully incorporated to avoid any clumps or dry spots in your casserole. This step sets the base for an even flavor distribution throughout the dish.
Spoon the mixed broccoli mixture into a prepared casserole dish, spreading it evenly to ensure consistent baking. Sprinkle crushed crackers over the top layer of the casserole, making sure they are distributed uniformly for an even texture and appearance. Drizzle melted butter evenly across the crackers to add richness and prevent them from becoming too dry during baking.
Place the broccoli casserole in the preheated oven and bake for 30 minutes, or until the top is golden brown and the casserole is heated through. Keep an eye on it towards the end to ensure it doesn't burn. You'll know it's done when you see a nice golden crust forming on top and the aroma of cooked broccoli and cheese filling your kitchen.
After baking, let the casserole cool for 15 minutes before serving to allow the flavors to meld together. This cooling period is crucial as it prevents any potential burns from hot steam escaping when you cut into it. Once cooled, you can easily slice or scoop portions and serve the dish at its best.