Thinly slice the lemongrass, roughly chop the Thai shallot, and slice the garlic to make pounding easier.
Add the lemongrass to a mortar and pestle and pound until starting to break down. Add shallots, garlic, and black peppercorns, and continue pounding to a coarse paste.
Transfer the paste to a bowl. Mix in fish sauce, oyster sauce, sugar, and oil until evenly combined.
Add the pork chops and toss to coat well. Cover and marinate in the fridge for at least 1 hour (up to 4 hours for deeper flavor).
Heat a grill or grill pan over medium heat. Grill the pork chops for about 3–4 minutes per side, until cooked through.