1. When ready to make kreplach, bring a large pot of salted water to boil.
2. Combine beef, onion (if using), and pepper in a bowl and mix well. On a heavily floured surface or using a hand-cranked pasta maker, roll out dough as thin as it will stretch. Cut dough into three-inch squares; you should have 32.
3. Place half a teaspoon of filling in the center of each square. Fold squares over into triangles and pinch edges closed. Drop finished kreplach into boiling water; stirring gently with a wooden spoon to keep kreplach separate, and let simmer for 20 minutes.
4. If you see kreplach sticking together in the pot, separate gently with the wooden spoon. After 20 minutes, remove from pot with a slotted spoon or wire skimmer and serve right away in hot soup, or freeze.