Steam the frozen broccoli for 10 minutes until tender; drain.
While the broccoli steams, melt 4 tablespoons of butter in a large skillet.
Saute the diced onion until tender then add the steamed broccoli.
Season with salt, pepper, onion powder and garlic powder.
Next, add the cream of mushroom soup (do not dilute) and mayonnaise and stir to combine.
Add the grated cheddar cheese and crushed crackers then stir to combine; add the filling mixture to a square baking dish.
Combine the remaining crushed crackers with the melted butter and mix to coat the crumbs. Add the buttery cracker as a topping to the casserole filling.
Bake 25 minutes until hot and bubbly.
Notes
Fresh broccoli can be used in this casserole but will need to steam a few minutes longer until tender.