Slice and boil zucchini and onion in salted water till fork tender, Drain.
in a casserole pan layer half of zucchini/onion. Mix sour cream and cream of mushroom soup seasoned with salt and pepper. Spread half of mixture on top of zucchini/onion. Sprinkle half of cheese on top. Layer the rest of zucchini/onion, top with rest of mushroom/sour cream mix and top with remainder cheese.
Bake in a 425 oven for 45 min to 1 hours until cheese is golden.