Preheat the oven to 350 degrees F (175 degrees C).
Place turkey in an even layer on the bottom of a 9x13 inch baking dish. Top with a layer of onion and a layer of green beans so that the turkey is no longer visible. Pour the condensed soup over the onion layer, then sprinkle with shredded cheese. Stir together the cubed cheese and mashed potatoes; spoon over the top of the casserole, and spread to cover.
Bake for 30 to 40 minutes in the preheated oven, until heated through.