Preheat over to 350 degrees
Add frozen broccoli to colander, and run warm water over broccoli to slightly defrost. Chop broccoli into bite size pieces and return to colander to drain excess water.
Finely chop onion and mushrooms and set aside
Cube Valveeta Cheese into 1 inch cubes
Saute onion over low heat for 5 minutes
Add cubed Valveeta to onion and continue to stir until cheese is fully melted
Add Mushrooms, Cream of Mushroom soup, and Garlic Powder to cheese mixture and stir to combine
Add Broccoli to cheese mixture and stir to combine.
Pour mixture to greased baking dish, top with Bread Crumbs, and bake for 30 minutes.