Cut very tips off of fresh green beans. (**You can cut beans in half at this point if you wish; i prefer to leave them whole.) Cook in large pot of boiling, salted water until tender, about 15 minutes. Drain beans and return to pot.
Cut onion into thin slices. Saute onion in 1/2 stick butter (or margarine). Add onion and butter to pot with green beans. In same pot, combine sour cream, flour and seasonings; mix lightly.
Pour into greased casserole. Sprinkle with grated cheese. Melt remaining 1/2 stick butter, toss with crumbs. Pour over casserole.