I also despise the three can green bean casserole. I try not to use any cans except for the occasional canned San Marzano tomatoes or a can of tuna. But we do like a green bean casserole sometimes, so I make my own. It's pretty easy.
I've tried a lot of the alternative green bean casserole recipes out there, and to be honest, some just go too far. Sure, they have green beans, but the end result is barely recognizable as a green bean casserole, and those expecting a GBC wouldn't be too happy.
So here's what I do. First, I buy Haricots Verts. They're green beans but longer and more tender. I simply steam them (or you could roast them) until they're as tender as I want them to be. I cut them after they're tender to a basic bite-size length. Set them aside. I also think that if you had to (for time or money's sake), using canned green beans wouldn't be the worst thing, especially if there is not another ingredient like sugar in the can, and if you used a good quality bean, not just a supermarket generic thing. This is the one ingredient I would allow to be from a can, if it was necessary. (It's surprising how many canned vegetables have sugar added, like canned peas!)
Then I make a homemade cream of mushroom soup. First, saute the mushrooms. Get baby portabella, or cremini, mushrooms, or white button mushrooms if you prefer, a couple of cups worth (they'll shrink down). Do not cut them or slice them. Saute them whole in a small amount of butter and olive oil until they're golden and tender. If you slice or cut mushrooms before sauteeing them, you will lose a lot of their moisture. After they're tender, chop them into bite-size or smaller pieces. Smaller pieces will more closely resemble the mushrooms in the canned soup. Then make a basic white sauce. About a half cup of butter or oil, melted, combined with an equal amount of flour, cook for several minutes until the flour is completely combined, then stir in about 4 cups of milk, stir until thick and creamy. Then add in all the mushrooms and stir gently to combine. The sauce should not be too thick, as a lot of it will be absorbed into the onions and beans. Set this aside.
Next, the onions. Get a couple of large sweet onions (or plain white onions). Peel them, and slice them as thinly as possible. I usually don't use whole rings, but I slice the onions in half and then slice them, so I end up with half-moon shapes. Soak the onions in buttermilk for an hour or so. Drain them. Into a big pan, like a cake pan, put a couple of cups of flour, and season it with some salt and pepper. Separate the onion half-rings with your hands, and place them into the flour. Make sure they're well coated. At this point, I don't fry them, just to keep them a little lighter. The secret to baking them is to use a large sheet pan, don't crowd the onions too much, and just drizzle them with a little oil. Toss them around on the pan so they all have a touch of oil on them and bake at about 400 degrees until they begin to get crunchy and brown. Toss occasionally so they cook evenly. Of course, you could fry them. They'll brown nicely if they're not crowded, so you might need to do them in batches. Separating them so they're not clumped is really important.
Then I just make the casserole like the traditional recipe directs, using some soy sauce (I use the Tamari gluten free soy sauce) or a little Worcestershire sauce, assembling it in the usual way. It's just that all the ingredients are homemade. And it looks like the canned one, but it has so much fresh taste and so far, everyone loves it! It doesn't stray from the traditional canned method, just every ingredient is vastly improved and you know exactly what's in it! Nothing fake!