Oh, it's a pretty straightforward, very lazy gravy! I make a roux with flour and oil, then add in broth while whisking vigorously to get rid of any clumps. Then I add a can or two of canned mushrooms and I season it with salt, pepper, soy sauce, maybe some msg, and herbs. Lots of herbs. Rosemary, sage, thyme, parsley, usually dried because that's what we have sitting around, but I bet fresh would be even better. We also have some, like, mushroom stir fry sauce (I don't exactly know what it is) that we found in an asian market, so I usually add some of that to get it more mushroomy.

And yeah, I've done fried onions on a green bean casserole. In my mind that's different than a tater tot hot dish. I do love green bean casserole though!