In a bowl, combine the lemongrass, garlic, salt, coriander, and pepper and stir to mix. Add the oil and stir until a thick paste forms. Set aside.
Place the pork loin on a cutting board. Beginning at one end of the loin, make a horizontal cut about 1-inch deep. Continue cutting the length of the loin. Then, return to the start of the cut and deepen the cut, pulling back the meat as you go, until the pork loin opens like a magazine. Do not cut all the way through.
Spread three-quarters of the spice mixture on the top of the pork and then roll the pork into a tight cylinder, securing it at 3-inch intervals with butcher’s twine. Rub the remaining mixture on the outside of the meat. Place the meat on a rimmed baking sheet, cover loosely with plastic wrap, and refrigerate for at least 8 hours.
When you are ready to cook the meat, remove it from the refrigerator and bring it to room temperature. Preheat the oven to 325 ° F.
Roast the pork for about 1 ¼ hours, until an instant-read thermometer inserted into the thickest part of the shoulder registers 140 ° F. Transfer the shoulder to a platter and tent with aluminum foil.
If serving the pork hot as a main course, let rest for about 30 minutes then slice the shoulder and serve. If you’re planning to use the pork for sandwiches, let cool completely before slicing.