Make the sweet and sour sauce by simmering brown sugar, vinegar, ketchup, soy sauce, pineapple juice, and Chinese five spice in a saucepan until the sugar dissolves, about 5 minutes.
Stir in the cornflour slurry and cook for 2 minutes until thick and glossy, then set aside.
Heat a large wok over high heat with vegetable oil, add meatballs, and cook for 4–5 minutes, turning until nearly done.
Stir in onion for 2 minutes, then add red capsicum and cook for another minute.
Add pineapple and the sauce, tossing everything until coated and heated through.
Remove from heat, fold in spring onions, and serve hot with steamed rice.