Preheat the oven to 350℉. Spray a 9x13-inch pan with nonstick spray.
In a large skillet, cook the beef and onion over medium heat until the meat is no longer pink; remove and discard the fat. Season with salt and pepper.
Transfer the meat mixture to the prepared baking dish. Top with the Tater Tots. In a medium bowl, whisk together the soup and milk. Pour over the potatoes. Sprinkle with the cheese. Bake, uncovered, for 40 minutes.