I know there are a few different varieties of green bean casserole. This is the one I use:
2 cans green beans (drained)
3/4 cup milk
1 can of cream of mushroom soup
1/8 tsp pepper
Mix together into casserole dish.
Stir in half of the onions into the casserole. Set aside the rest for later.
Bake uncovered at 350 for 30 minutes.
Remove casserole after 30 minutes and top the rest of the onions onto the casserole. Bake 5 more minutes.