Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13 inch-baking dish. Arrange tater tots in the prepared baking dish. In a mixing bowl, combine sour cream, cheese, and mushroom soup.
Pour this mixture over the tater tots. Sprinkle onions over the top of the casserole. Bake in the 350 degrees F (175 degrees C) oven for 45 to 60 minutes