For the sauce: Combine the sugar, corn flour, vinegar, lemon juice, tomato sauce in a bowl, mix, and set aside.
In another bowl, add the sugar and fish sauce, then stir until sugar dissolves.
Add the pork followed by the black pepper and marinate for 2 hours.
Place a hot wok on high heat; add 1 tablespoon oil followed by garlic and pork.
Stir-fry for 3 minutes then take out and set aside.
In the same hot wok add remaining oil, tomato, capsicum, and pineapple, and stir-fry 3 minutes.
Return the pork back to the wok then pour in the prepared sauce.
Stir, then add onions, spring onions, and Asian celery and stir-fry for a further 2 minutes until the sauce thickens.
Serve with jasmine rice with a side bowl of chile and light soy sauce for dipping.