This has been an old favorite.  Chances are this is the original recipe off the soup can.

Green Bean Casserole Recipe

• enough butter or olive oil to saute (2-3 tablespoons)
• 1 small onion chopped
• 1 small box sliced fresh mushrooms
• 2 cups fresh green beans
• 3 cups chicken broth
• 1 can of cream of mushroom soup
• 2 cans french fried onion rings
• Salt and Pepper

Cook green beans in chicken broth until tender.  Drain

Saute onion and mushrooms until soft.  

Mix 1 can of onion rings with all other ingredients and pour into a baking dish.  Bake 20 minutes at 350, then top with more onion rings.  I use the whole second can but some use less.  Also, some put cheddar cheese on the top but we always just stuck to the onion rings.  Return to oven for 10 more minutes.