Preheat air fryer to 350°. In a large skillet, cook beef and onion over medium heat until meat is no longer pink; crumble beef; drain. Season with salt and pepper. , Transfer to a greased 2-qt. baking dish that will fit in the air-fryer basket. Top with Tater Tots. Combine soup and milk; pour over potatoes. Sprinkle with cheese. Place baking dish on tray in air-fryer basket. Cook, uncovered, until heated through, 30-40 minutes.