For the sweet & sour sauce, place brown sugar, tomato ketchup, soy sauce, vinegar, pineapple juice and Chinese five spice in a saucepan over medium-high heat. Simmer for a few minutes until sugar dissolves. In a small bowl, combine cornflour with 2 tablespoons of water. Add the cornflour mixture to the sauce and simmer for a further 2 minutes to thicken. Remove from heat and set aside.
In a large bowl toss chicken with soy sauce, garlic and pepper until well combined.
Place flour in a large bowl. Add the chicken and toss until each piece is lightly coated with flour.
Pour about a 3 cm (1 inch) depth of oil into a large non-stick frying pan over high heat. When the oil is hot (a piece of chicken should start sizzling immediately when placed into the oil) cook chicken in batches for 2-3 minutes each side or until golden brown and cooked through. Drain on paper towel.
Heat the remaining 1 tablespoon of oil in a clean frying pan or wok. Add the onion and stir-fry until just starting to turn golden. Add the tomato and stir-fry for another 30 seconds. Add the chicken and the sweet and sour sauce and stir-fry until well combined. Toss through the spring onions. Remove from heat and serve.