Sour Cream Green Beans

1 onion, thinly sliced
1/2 cup butter or margarine, divided
2 (10 ounce) packages frozen green beans, thawed and drained
1 cup dairy sour cream
1/4 cup flour
1 1/2 teaspoons salt
1/4 teaspoon pepper
1 cup shredded sharp natural Cheddar cheese
1 cup soft bread crumbs

Saut