Swiss cheese green bean casserole

1. Sauté onion in the 3 T. of butter in a large frying pan.
Blend flour, salt & pepper with the sautéed onions in the frying pan, and stir to coat all the flour in the butter.
Add milk and stir until thick and bubbly.
Stir in green beans until well coated.
Stir in Swiss cheese until it becomes melty.
Transfer the whole thing to a casserole dish.
Combine the crushed cornflakes and melted butter in a small bowl, and then sprinkle over casserole.
Bake at 400 degrees for 20 minutes (can be baked at 350 degrees if necessary, for about 40 minutes).