Slice the pork into 1 cm thick rounds. Finely chop the onion.
Heat a spoonful of oil in a wok and stir-fry the pork for 2-3 minutes, or until golden. Transfer to a plate and keep form.
Stir-fry the onion for 2 minutes then add the sugar, fish paste and 3 Tbsp water. Cook until you get a thick sauce.
Return the pork and cook for a couple of minutes to glaze the slices, then add 1 tsp of Siracha sauce - more if you want it very spicy. Taste, season and add some lime juice.
Serve hot with steamed Jasmine rice.