STUFFING: In skillet, heat oil over medium heat: cook onions, garlic, red chili pepper, tomato and gingerroot, stirring often, for about 5 minutes or until onions are softened. Stir in cashews, salt and pepper; let cool. (Stuffing can be refrigerated in an airtight container for 1 day.)
LEMONGRASS GLAZE: In small saucepan, bring water, sugar, cornstarch and lemongrass to boil over medium heat, stirring constantly, for 2 minutes or until thickened and clear. Strain through fine sieve into small bowl; let cool completely. (Glaze can be refrigerated in an airtight container for up to 1 day, reheat over medium-low heat until fluid.).
Trim tenderloins. Cut in half horizontally almost but not all the way through; open out flat. Evenly spoon stuffing lengthwise along centre of each. Fold thin ends over filling, then fold up sides to enclose filling. With 12-inch soaked wooden skewers sew each closed at 2-inch intervals. (Pork can be prepared to this point, wrapped in plastic wrap and refrigerated for up to 8 hours; let stand at room temperature for 30 minutes).
Sprinkle pork with salt and pepper; brush with half the glaze. Place on greased grill over medium heat; close lid and cook, turning and brushing occasionally with remaining glaze, for about 35 minutes or until juices run clear when pork is pierced and just a hint of pink remains inside. Transfer to cutting board; tent with foil and let stand for 10 minutes. Remove skewers; cut into 1-inch thick slices.