Preheat oven to 400 degrees F.

Add green beans to a large pot, and just cover with water. Bring to a boil and cook for 6 to 8 minutes, or until nearly tender. Remove the beans to a shallow casserole and toss with salt and pepper. Let the water in which the beans cooked reduce to a few tablespoons. Add the butter and let it melt. Add the flour and cook, stirring constantly, until thickened. Cover the beans with a layer of onions, then pour the sauce over the onions. Pour the cream over the sauce and add Swiss cheese. Stir to combine. Sprinkle the casserole with buttered breadcrumbs. Bake for 20 to 30 minutes or until golden on top.