Marinate the pork: In a small bowl, mix together the minced garlic, soy sauce, lime juice, brown sugar (if using), and fish sauce (if using). Rub the mixture all over the pork loin, making sure it’s evenly coated. Let the pork marinate for at least 30 minutes, or up to 2 hours in the refrigerator.
Prepare the grill: Set up your charcoal grill for two-zone cooking: pile the lit coals on one side of the grill to create a hot direct-heat zone, and leave the other side for indirect heat. This allows you to sear the pork first and then finish cooking it slowly without burning.
Grill the pork:
- Searing: Brush the pork loin with a little neutral oil to prevent sticking. Place the pork over the hot direct-heat zone and sear for 2-3 minutes per side, until nicely charred and browned.
- Finishing: Move the pork to the indirect heat zone and continue grilling, turning occasionally, until the internal temperature reaches 145°F (63°C). This should take about 10-15 minutes depending on the thickness of the pork.
Rest and slice: Once the pork reaches the correct internal temperature, remove it from the grill. Let it rest for 5-10 minutes, covered loosely with foil, to allow the juices to redistribute. Slice the pork thinly for serving.
Serve alongside the tom kha: Serve the char-grilled pork loin with your tom kha, and optionally garnish with fresh lime wedges or coriander.