Place the pork in a bowl. Add half of the sugar, fish sauce and black pepper. Mix well, working the seasonings into the meat. Cover and marinate in the fridge for at least two hours.
Crush and mince the garlic.
Roughly chop the onion and bell pepper.
Cut the tomato into quarters (or eighths, if it is rather large).
Finely slice the celery stalk.
Roughly chop the onion leaves.
Mix together the remaining sugar, starch, vinegar and lemon juice for the sauce.
Heat two tablespoons cooking oil in a wok or frying pan.
Add the pork and stir fry (see stir frying basics) until the meat changes color.
Add the garlic and continue stir frying until the edges of the pork start to brown lightly.
Scoop out the pork and keep warm.
Add the remaining cooking oil to the wok and heat. Stir fry the onion, celery, bell pepper and tomato.
Return the pork back into the wok.
Add the pineapple chunks.
Pour in the sauce.
Cook, stirring often, just until the sauce thickens.
Transfer the stir fried dish to a platter. Garnish with onion leaves and the celery leaves (and cilantro, if you like).
Serve your Vietnamese pork and pineapple stir fry with hot rice.