Preheat your oven to 350 degrees F (175 degrees C).
In a large bowl, combine the melted butter, condensed cream of mushroom soup, milk, chopped onion, salt, and pepper. Stir until well combined.
Add the thawed and drained broccoli to the bowl. Mix gently to coat the broccoli with the soup mixture.
Stir in the shredded cheddar cheese until it is evenly distributed throughout the broccoli mixture.
Pour the broccoli and cheese mixture into a 9×13 inch baking dish.
In a small bowl, combine the crushed buttery round crackers with 2 tablespoons of melted butter. Sprinkle this cracker topping evenly over the broccoli mixture in the baking dish.
Bake for 30 to 40 minutes, or until the casserole is bubbly and the cracker topping is golden brown.
Let the casserole stand for 5 minutes before serving.
Notes
For a make-ahead option, prepare the casserole up to the point of adding the cracker topping. Cover and refrigerate for up to 2 days. Add the cracker topping just before baking and increase the baking time by 5-10 minutes.
You can freeze the unbaked casserole for up to 3 months. Thaw in the refrigerator overnight before baking.
Ensure your broccoli is well-drained to prevent a watery casserole.
Use real sharp cheddar cheese for the best flavor.