Coat a 2-qt baking dish with nonstick cooking spray.
In a large bowl, mix soup, sour cream and onion. In a medium bowl mix spinach, nutmeg and 3/4 cup of the soup mixture. Add the sweet potatoes to the remaining soup mixture.
Layer 1/2 of the potatoes over the bottom of the baking dish. Sprinkle with 1/3 of the cheese and top with all of the spinach mixture, then 1/2 of the remaining cheese, and all the remaining potatoes. Cover the dish tightly with foil.
Bake for 1 hour and 20 minutes or until potatoes are tender when pierced.
Uncover and sprinkle with remaining cheese and bake for an additional 5 minutes to melt the cheese.