In a large saucepan, melt the butter. Add the flour and whisk until the mixture becomes fragrant and nutty, about 1 minute.
Slowly whisk in the chicken broth and milk.
Bring mixture to a boil, whisking constantly, until thickened. Season with salt and pepper. Drain the liquid from the cans of green beans and add them to the cream sauce.
Pour beans and sauce into a lightly greased 13x9-inch baking dish. Sprinkle the top with French fried onions, and bake at 350 degrees Fahrenheit for 20 minutes, until bubbling in the center and heated through.
Video
Notes
*Sauté some sliced mushrooms in butter, and add them to the sauce for a more traditional green bean casserole.