Prepare the pork balls: In a large bowl, combine the ground pork, breadcrumbs, egg, garlic, ginger, soy sauce, sesame oil, sugar, salt, pepper, cilantro, and chopped onion. Mix well until all ingredients are evenly incorporated.
Stuff the pork balls: Take a small portion of the pork mixture and flatten it into a patty. Place a small amount of shredded mozzarella cheese (if using) in the center, then fold the edges of the patty over the cheese to form a ball. Repeat with the remaining mixture.
Fry the pork balls: Heat vegetable oil in a large skillet over medium heat. Once hot, add the stuffed pork balls in batches, being careful not to overcrowd the pan. Fry them for about 5-7 minutes, turning occasionally, until golden brown and cooked through. Remove and set aside.
Stir-fry the bell peppers: In the same skillet, add a bit more oil if needed. Add the sliced bell peppers and stir-fry for 2-3 minutes until they are slightly tender but still crisp.
Combine the pork balls and peppers: Return the cooked pork balls to the skillet with the bell peppers. Add soy sauce, oyster sauce (if using), and rice vinegar (if using). Stir everything together and cook for an additional 2-3 minutes, allowing the flavors to meld.
Serve: Transfer the stir-fried stuffed pork balls with bell peppers to a serving dish and garnish with additional cilantro if desired. Serve hot with steamed rice or noodles. Enjoy!
