Preheat oven to 350 degrees F (175 degrees C).
Mix cream of mushroom soup, water, turkey, wild rice, milk, and onion together in a 2-quart casserole dish. Cover dish with aluminum foil.
Bake in the preheated oven for 50 minutes. Remove foil and cook until rice is tender and liquid is absorbed, 10 to 15 minutes more.