For the Meatballs:

1. In a mixing bowl, combine the ground turkey, breadcrumbs, milk, minced onion, minced garlic, egg, soy sauce, salt, black pepper, and red pepper flakes.
2. Mix the ingredients until well combined.
3. Shape the mixture into small meatballs, about 1 inch in diameter.
4. Heat the vegetable oil in a large frying pan over medium heat.
5. Add the meatballs to the pan and cook until browned on all sides and cooked through, approximately 10-12 minutes. You may need to do this in batches to avoid overcrowding the pan. Remove the meatballs from the pan and set them aside.

For the Sweet and Sour Sauce:

1. In the same pan, add a bit more oil if needed.
2. Add the diced onions and bell peppers to the pan and sauté for about 3-4 minutes, or until they become slightly tender.
3. In a small bowl, mix the cornflour with a couple of tablespoons of water to make a slurry.
4. In a separate bowl, combine the ketchup, white vinegar, brown sugar, pineapple juice, soy sauce, and cornflour slurry.
5. Pour the sauce mixture into the pan with the sautéed vegetables.
6. Stir well and bring the mixture to a simmer.
7. Add the cooked turkey meatballs and pineapple chunks to the sauce.
8. Simmer for an additional 5-7 minutes, or until the sauce thickens and the meatballs are heated through.
9. Season with salt and pepper to taste.

Serve your sweet and sour turkey meatballs over rice or noodles for a complete meal, or as an appetiser with toothpicks for a tasty party treat. Enjoy!