In a blender or food processor, puree onion, garlic, ginger, and lemongrass. Add 2 tablespoon water to get blender or food processor going if necessary. Transfer to a bowl. Add curry powder, ground cumin, ground fennel, and chili powder. Mix to form a paste.
Carefully cut off excess fat or membrane if any. Then cut pork roast into ¼ inch thick slices to about ½ inch from the base. Do not cut through so that pork roast remain as a single chunk. Sprinkle ground pepper all over the pork roast.
Heat 2 tablespoon vegetable oil in a medium cast iron pot. Place pork roast cut side up and brown for 3 minutes.
Using tongs and spatula carefully flip pork roast cut side down and continue to brown for another 3 minutes. Remove pork roast from pot and set aside.
Add remaining 2 tablespoon vegetable oil into the cast iron pot. Add spice paste and kaffir lime leaves. Stir fry over medium heat until fragrant, about 4 to 5 minutes.
Return browned pork roast cut side up into the cast iron pot. Spoon curry paste over the top of pork roast.
Pour in 2 cups (480ml) water. Cover and bring to a boil. Reduce heat and simmer for 40 minutes.
The curry gravy would have been reduced considerably. Add remaining ½ cup (120ml) water and season with salt.
Add cubed potatoes and continue to simmer for another 15 minutes.
Finally, add coconut milk and allow it to heat through, about 5 minutes.
Turn off stove and allow curry to sit for 10 minutes before serving.