Heat the oil in a large non-stick frying pan over a medium heat and fry the lemongrass, chilli and garlic for 2-3 minutes until fragrant. Add the pork mince and cook over a high heat for another 6-8 minutes, stirring all the time, until the mince is browned all over. Drain off any excess liquid if necessary.
Add the palm sugar and fish sauce and let the mixture bubble for 2-3 minutes, then stir in the lime juice and nearly all the coriander.
Cook the rice according to the instructions on the packet, then stir it into the pork. Season to taste with freshly ground black pepper.
Divide the pork larb between the lettuce leaves and scatter over the rest of the coriander. Garnish with cucumber matchsticks and sliced chilli.