Preheat your oven to 350°F and grease a 9x13-inch baking dish.
In a large bowl, combine the thawed broccoli, cream of mushroom soup, sour cream, 1 1/2 cups of the shredded cheese, mayonnaise, diced onion, beaten eggs, garlic powder, salt, and pepper. Mix until everything is well combined.
Pour the broccoli mixture into your prepared baking dish and spread it out evenly.
In a small bowl, combine the crushed Ritz crackers with the melted butter. Toss until the crackers are well coated.
Sprinkle the remaining 1/2 cup of cheese over the broccoli mixture, then top with the buttered cracker crumbs.
Bake for 35 minutes, or until the casserole is bubbly around the edges and the cracker topping is golden brown.
Let it cool for 5 minutes before serving – the flavors will settle and it'll be easier to scoop!