Wash and pat dry pork shoulder or loin with paper towels. With a sharp knife, trim off fat and cut into 1 cm thick slices. (note 1)
Finely chop lemongrass, onions, spring onions and garlic. For best results, grind to a rough paste with a pestle and mortar. Or, roughly chop and blend in a small food processor. (note 2)
In a large bowl or ziplock bag, combine all marinating ingredients and mix well. Add pork slices and coat well (if using a ziplock bag, seal then rub pork all over). Cover the bowl with cling wrap and refrigerate for 1 hour to marinate.
When ready to cook, add 1 tablespoon of oil to a large non-stick pan or griddle pan on medium-high heat. Once the pan is hot, with a pair of tongs, place pork slices onto the pan and cook for about 2 minutes per side or until cooked through and pork is lightly charred on both sides. Do not overcrowd the pan. Make sure there is wiggle room in between each slice. You most likely will have to cook in 3 batches.
Once the first batch is cooked, wipe clean the pan with a scrunched up paper towel using a pair of tongs. This will get rid of charred bits of aromatics from the first batch.
Repeat for 2 more times or until all pork have been cooked.
Serve with some rice, fresh tomato and cucumber slices and a side of nuoc mam as a dipping sauce.