Broccoli Casserole

2 10 ounce boxes frozen broccoli (whole or chopped)
1/2 cup chopped onion (divided)
2 tablespoons margarine or butter
1 can cream of mushroom soup
1/2 cup milk
1/4 cup grated sharp cheese
6 salted crackers

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Boil broccoli with 1/4 cup onion until tender. Drain and place in casserole dish. Mix grated cheese and 1/4 cup onion. Add soup, mixed with 1/2 cup milk. Pour mixture over broccoli. Roll crackers and mix with melted margarine. Sprinkle on top of casserole. Bake at 350 degrees for 25 to 30 minutes or until brown and bubbly.