Preheat your oven to 350°F (175°C). Lightly grease a 9×13 inch (23×33 cm) baking dish with butter or cooking spray.
Steam or blanch the broccoli florets until tender-crisp, about 3-5 minutes. They should be bright green but still firm. Drain thoroughly and pat dry to remove excess water.
In a large mixing bowl, whisk together the cream of chicken soup, mayonnaise, melted butter, and eggs until smooth and well combined.
Stir in the finely diced onion, salt, and black pepper. Taste the mixture and adjust seasoning as needed, remembering the cheese will add saltiness.
Gently fold in the steamed broccoli florets and 1.5 cups (165g) of the shredded extra sharp cheddar cheese. Ensure all ingredients are evenly coated.
Pour the mixture into your prepared baking dish, spreading it out evenly.
In a small separate bowl, crush the Ritz crackers and mix them with the remaining 1/2 cup (55g) of shredded cheddar cheese.
Sprinkle this buttery, cheesy topping generously over the broccoli mixture in the baking dish.
Bake for 30-35 minutes, or until the casserole is bubbling around the edges and the topping is beautifully golden brown and crispy. If the topping browns too quickly, you may loosely tent the dish with foil.
Remove the casserole from the oven and let it rest for 5-10 minutes before serving. This allows the casserole to set, making it easier to serve.