In my student days, I used to get a home brand cheese sauce sachet, big tin of tuna and a tin of corn and make the cheese sauce and mix the tuna and corn in, then put bread crumbs and cheese on top and bake. Was quick because you really only needed to heat it through, and only cost a few dollars to make, would last me a few days. Sometimes I would mix pasta in too (pre cooked) or frozen peas. Kind of like a tuna mornay I guess, so not sure if it is the type of recipe you were after, I'm not sure whether it counts as a casserole!