Instead of canned soup in casserole...
Pretty much any variation on a bechamel sauce will work. I make cheesy rice/vegetable type casseroles this way quite often. If you need a basic recipe for the sauce I can post one, but Joy of Cooking or somthing comparable should be able to get you started.
P.S. The sauce is even better if you add shallots to the butter in the beginning.
my partner has revolutionized tuna casserole by figuring out just such a sauce. to replace canned cream of mushroom soup, she makes a bechamel sauce and adds mushrooms which have been dry-sauteed.
the resulting casserole is really wonderful, with a richer flavor than the "canned' version. we still use canned tuna, though. I think using seared ahi would kind of kill the concept.
to dry-saute mushrooms, wipe and quarter the mushrooms. heat a trusty cast iron skillet over high heat and when it is hot, add the mushrooms and stir-fry. they will squeak and sizzle. gradually they will release a surprising amount of moisture, then reabsorb it. now they are ready to be added to the casserole along with your sauce and onions.
i think the canned soup was itself a replacement for the "white sauce" craze of the early 20th century, when you could even buy cans of the stuff. Ah, domestic scientists.