1) For the salsa, combine all of the ingredients in medium mixing bowl and chill until ready to serve.
2) For the fillet, use a sharp knife to trim fillet of any fat by removing as much of the silver skin as possible.
3) Combine watermelon and jalapenos in a blender and puree, about 2-3 minutes. Strain mixture through fine sieve and return to blender. Add the salt, sugar and melon liqueur (if using) to blender and blend together, about 2-3 minutes. Reserve 225ml of marinade for basting. Draw remaining marinade with syringe injector. Inject pork liberally with marinade. Sprinkle salt and pepper to cover pork. Let marinate while heating the griddle.
4) Remove pork from platter and place directly on the flame for direct heat. Roll the pork over the direct heat to give it griddle marks. Remove pork from direct heat and place away from the flame to finish cooking. Baste with glaze every 10-15 minutes until pork reaches an internal temperature of 62C. Remove from griddle to platter and let cool for 5 minutes.
Slice pork into medallions and serve with watermelon salsa.