I can't say I ever make a casserole dedicated to zucchini, but I include it in stirfries and spaghetti.
I guess spaghetti counts as a casserole, at least the way I do it. For me, it's not just pasta topped with sauce. The sauce is the major element, because it contains a number of veggies and cheese and sometimes meat. So use any spaghetti sauce, even Ragu. Chop up and saute your zucchini along with onions, peppers, tomatoes, broccoli and whatever you feel like adding. Then add your tomato sauce/paste/seasonings. Cook and add in beef if you want. Grate on some cheese. Serve over the pasta. I prefer spirals or elbows, and whole wheat to enriched white noodles.