Preheat oven to 450 degrees F and grease a 9x13 inch baking dish with cooking spray. Set aside.
Melt 1/2 cup butter in a non-stick skillet over medium heat.
Add in onions and cook until tender, about 5-10 minutes.
Pour into a large mixing bowl, then add in mushroom soup, mayonnaise, sour cream, egg, salt, pepper, paprika, and garlic powder.
Whisk until combined.
Stir in broccoli and cheese.
Pour into prepared baking dish. Set aside.
In a medium sized bowl, stir together crushed cheez-its and 2 tablespoons melted butter.
Sprinkle evenly over casserole.
Bake in preheated oven for 25 minutes, or until heated through.
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