Saute onion in margarine until it is transluscent.
Add broccoli and cook until thawed.
Add remaining ingredients, except grated cheese. Cook until cheese is melted.
Pour into a greased 9x13-inch casserole dish. Sprinkle grated cheese over top of casserole. Bake at 350 degrees for 15 minutes or until cheese is melted and casserole is hot and bubbly.
I usually make the casserole the night before and let it sit covered in the refrigerator overnight before baking. If you do this, it takes more than 15 minutes to bake. Just bake it until the casserole is bubbling and the cheese on top is melted.