In a large skillet, cook beef and onion over medium heat until meat is no longer pink, 5-7 minutes; crumble meat; drain. Add sauerkraut and cheese.
Preheat oven to 350°. Unroll crescent roll dough and separate into rectangles; pinch seams to seal. Place 1/2 cup beef mixture in the center of each rectangle. Bring corners to the center and pinch to seal. Bake until golden brown, 12-15 minutes.