I'm not including the recipe directly on this page, as I didn't really change it. So I'm giving you the link to the Smitten Kitchen webpage and you can take advantage of her lovely photographs. (We can blame Alton Brown for the original idea about the onions; see SK's commentary on his version.) I would advise that you make 25-50% more sauce and more onions. You can't have too many onions for this! (Just be sure that you stop cooking them before they turn too dark, as they continue to cook after you remove them from the heat.) It's not that hard, really. Just try it once and you'll never go back. (The picture above is the only one I got around to taking. It's the FRESH mushroom sauce.)
If you say, "No way am I going to make my own French-fried onions!" then you can make the America's Test Kitchen version, which you can access here: http://www.food.com/recipe/americas-test-kitchen-green-bean-casserole-421411. (ATK allows free access onto to its current season, but their recipes are posted all over the web, and this site seems legit.) It's very, very good--but the one made with the homemade onions is transcendent. Your choice.