A few notes:

I made a half-recipe of this dish (in a 9-inch skillet) as there are only three of us. Yours will be bigger. It was so good, we wished we had more.
I recommend a range of quantities for the green beans; if you like your casserole to be heavier on sauce and lighter on vegetables, use the lower amount. If you’d like a more vegetable-heavy dish, light-to-moderately coated with the sauce (as mine is in the photos), use the higher amount.
This makes a loooot of crispy onions. We didn’t mind. But you could probably stretch half the onion volume more thinly over your whole casserole (saving the other half for snacking/sandwiches), or you could just say, “It’s Thanksgiving. Let’s do this,” and use them all. Either way, you win.
[Apologies, this was originally in the recipe and I'd meant to make it a footnote due to the unevenness of the results.] If you’d like to bake the onions instead of frying them, Alton Brown recommends spreading them on 1 to 2 oiled baking sheets (but I’d use parchment paper next time) and baking them at 475 (but I might try a lower temperature) for approximately 30 minutes (though I’d check it at 20 minutes), tossing them around a few times to ensure even cooking. However, I found this a little pesky — it’s much harder to get them crisp and golden and they’re prone to getting too dark before most of them are cooked. I ended up deep-frying the rest and have zero regrets. It’s Thanksgiving. Let’s do this properly.
About the sauce thickness: Alton’s original sauce was on the thin side (2T butter for 2T flour). I thickened it when I made it so that it is a moderately thick sauce — it coats the beans (as you can see in the photos) but it does drape a little bit once baked (i.e. there will be more on the lower half of the pan than the top half when it comes out of the oven). If you’re nervous and want to make sure that your sauce is definitely very thick, you can do so by using 1 extra tablespoon butter and 1 extra tablespoon flour in the sauce — i.e. more roux makes thicker sauces. Draining your green beans well and even patting them out on towels will help ensure they don’t “liquefy” the sauce too much.