I'm not sure it counts as scalloped but I have a good recipe for a casserole thing. So easy.
mix together: a bag of southern style frozen potatoes a cup of melted butter a can of cream of mushroom (or potato or chicken) soup 8 oz of sour cream 1/2 an onion, chopped and as much cheese as you can stand. Then add more cheese to cover the top and bake at 375 for 45-60 minutes (depending on how brown you like your cheese)
My sister makes a similar recipe, but does not cook the sauce, rather she layers and pours milk and flour into the layers. Hers is always delicious also.