Preheat oven to 350 degrees, spray baking dish with non stick cooking spray.
Clean broccoli and cut into bite sized pieces.
Place chopped broccoli in a microwave-safe bowl. Pour ¼ cup water over the top. Cover with a damp paper towel and microwave on high 2 ½ to 3 minutes. Drain well.
In a large mixing bowl add the mushroom soup, drained mushrooms (if using), mayonnaise, eggs, onions and cheddar cheese. Mix well to combine. Add the steamed broccoli and mix again. Pour into prepared baking dish.
In a small mixing bowl, combine melted butter, crushed crackers and parmesan cheese. Sprinkle cracker topping over the top of the casserole.
Bake for 35-40 minutes until the casserole is set and the top is golden brown.
Notes
The parmesan cheese and sliced mushrooms are optional, but they both add really great flavor and texture to this simple casserole dish. You can substitute with 3 boxes frozen broccoli, thawed and drained well. If you do, you can omit steaming the broccoli.