Preheat the OvenPreheat your oven to 350°F (175°C). If you're using fresh broccoli, bring a large pot of water to a boil and blanch the broccoli florets for 2-3 minutes until they are bright green and slightly tender.
Drain and set aside. For frozen broccoli, simply thaw it according to package directions and drain any excess water.
In a mixing bowl, combine the cream of mushroom soup, mayonnaise, sour cream, milk, chopped onions, garlic powder, and black pepper. Whisk until all ingredients are well blended and smooth.
In a large bowl, add the blanched or thawed broccoli and the creamy mixture. Stir gently to coat the broccoli evenly with the sauce. Transfer this mixture to a greased 9x13-inch baking dish.
Sprinkle the shredded cheddar cheese evenly over the top of the broccoli mixture.
In a small bowl, combine the crushed Ritz crackers or breadcrumbs with the melted butter. Mix until the crumbs are well coated with the butter. Sprinkle this mixture evenly over the cheese layer in the casserole.
Place the casserole in the preheated oven and bake for 25-30 minutes, or until the top is golden brown and the casserole is bubbling around the edges.
Remove the casserole from the oven and let it cool for about 5 minutes before serving. Enjoy it as a comforting side dish or a vegetarian main course!