Preheat the oven to 177 degrees C (350 degrees F), use a sharp knife to slice the mushrooms into smaller pieces, then set aside. Place a non-stick skillet pan over medium heat, then add in the butter. Once the butter melts, add in the mushrooms and cook for about five minutes until they become soft. Add in the parsley, salt, and pepper to taste then stir to combine.
Next, add in the heavy cream and mustard, decrease the heat to low then simmer for about five to ten minutes until the sauce reduces, set aside. Add some sprinkles of salt and pepper over the fish to taste then add to a baking dish greased with cooking spray.
Add in half of the cheese followed by the cooked mushrooms then the rest of the cheese. Place the baking dish into the preheated oven and bake for about thirty minutes until the fish is cooked through.
Use a sharp knife to cut the broccoli into small florets, place a cooking pot over medium heat, add in water, salt, and the broccoli then boil for a few minutes until the veggie is cooked. Drain off the water, place the broccoli in a bowl, add in the butter, salt, and pepper to taste then mash.
Serve the fish casserole with the broccoli and enjoy.