If you have a pork tenderloin, cut it into diagonal slices about 1/2 inch thick.
Make a marinade by adding the cilantro, scallions, ginger, garlic, lemon zest, chile, and lemongrass into a food processor and whiz till you have a paste. Put in a bowl and add in the rest of the marinade ingredients. If using a blender just blend everything together. Place the pork in a dish and mix through the paste and leave to marinade for half an hour or even overnight.
Heat a frying pan with no oil and add the pork (keep any remaining marinade) in a single layer, or as close as you can to a single layer, for a minute or two each side until golden. Don't worry about the marinade getting all burned and sticky in the pan! When the pork is golden remove from the pan and keep warm.
While the pork is cooking, place reserved marinade in a small saucepan. Add in the coconut milk, bring to a boil and boil uncovered for about 5 minutes, until it has thickened a bit. Add the fish sauce, lemon juice and chopped cilantro. Season to taste with more lemon juice and fish sauce if needed.
Add the pork back into the sauce and heat through for a few minutes. Serve with jasmine rice and a little cilantro on top!