Green Bean Casserole

2 TBS butter
2 TBS flour
1 tsp salt
1 tsp white sugar
1/4 cup onion, diced
1 cup sour cream
3 cans French style green beans (I used fresh/frozen green beans-enough to fill a 9X9 pan)
2 cups shredded cheddar cheese
1/4-1/2 cup fresh chopped mushroom
1/2 cup crumbled buttery round crackers (frozen onion rings or French's fried onions)
1 TBS butter, melted
Fresh grated parmesan cheese
pepper to taste.

Preheat oven to 350F. Melt 2 TBS butter in large skillet, stir in flour until smooth. Cook 1 minute. Add salt, pepper, sugar, onion, mushroom and sour cream and parmesan cheese. Set aside. Boil fresh or frozen green beans until done. Drain and add green beans to cream sauce, stir to coat. Transfer to 9X9 baking dish. Cover with shredded cheddar cheese. If you use crackers, crush and mix with remaining butter. Add them now and bake for 3o min. or until bubbly. If you use onion rings, place them on top of cheese and bake for 30min. If you use fried onions, bake cheese topped casserole for 25 min. and add them the last 5 min. to brown.