*Prepare the Meatballs:**
- In a large mixing bowl, combine the ground beef, breadcrumbs, milk, egg, onion, garlic, soy sauce, salt, pepper, and ginger powder (if using).
- Mix the ingredients until well combined but do not overwork the meat.
- Form the mixture into 1-inch meatballs using your hands or a meatball scooper.
- Heat the vegetable oil in a large skillet over medium heat.
- Add the meatballs to the skillet in batches, cooking them until browned on all sides, about 5-7 minutes. They do not need to be fully cooked through as they will simmer in the sauce.
- Remove the meatballs from the skillet and set them aside on a plate lined with paper towels to drain excess oil.
**Make the Sweet and Sour Sauce:**
- In the same skillet (drain excess oil if necessary), add the ketchup, rice vinegar, brown sugar, and soy sauce. Stir well to combine.
- Bring the mixture to a simmer over medium heat, stirring occasionally.
- Add the cornstarch slurry to the sauce, stirring constantly until the sauce thickens, about 2-3 minutes.
- If using, add the pineapple chunks and bell pepper to the sauce and cook for an additional 2-3 minutes until the vegetables are slightly tender.
**Combine and Simmer:**
- Return the browned meatballs to the skillet, stirring gently to coat them with the sweet and sour sauce.
- Reduce the heat to low and cover the skillet.
- Simmer the meatballs in the sauce for 10-15 minutes, or until the meatballs are fully cooked through and the flavors have melded together.
**Serve:**
- Serve the sweet and sour meatballs hot, garnished with chopped green onions or sesame seeds if desired.
- Pair them with steamed rice, noodles, or even on their own as a delicious appetizer.