In a large skillet, cook beef and onion over medium heat until meat is no longer pink, crumbling meat, 5-7 minutes; drain. Add sauerkraut and cheese.
Preheat air fryer to 350°. Unroll crescent roll dough and separate into rectangles; pinch seams to seal. Place ½ cup beef mixture in the center of each rectangle. Bring corners to the center and pinch to seal. In batches, place turnovers in a single layer on a greased tray in air-fryer basket. Cook until golden brown, 12-15 minutes.
