A filling of pork tenderloin, mango, jicama, watercress, and basil is paired with a traditional peanut sauce flavored with sesame oil, garlic and minced chiles.
Make marinade: Combine fish sauce, canola oil, rice-wine vinegar, lemon grass, garlic, ginger, chile pepper, and black pepper in a medium bowl. Transfer to a resealable plastic bag; add pork, and refrigerate 3 hours or overnight.
Preheat grill or grill pan to medium. Remove pork from marinade. Place on grill; cook, turning as necessary, until a meat thermometer registers 160 degrees. Remove from grill; let cool. When cool enough to handle, slice meat 1/8 inch thick; set aside.
Dip a rice-paper wrapper into a bowl of warm water for 5 seconds; transfer to a clean work surface (wrapper will still feel hard but will soften as it sits). Lay watercress on bottom third of rice paper. Top with some pork, mango, jicama, and basil. Roll (you needn't tuck in ends). Place on a plate; cover with a damp paper towel. Continue filling and rolling rice paper until all ingredients are used. Trim ends; halve. Halve each half again on the diagonal to make hors d'oeuvres; serve.
To create bite-size hor d'oeuvres, the rolls can be cut in half, and then halved again. Don't worry about tucking in the ends of the rice-paper skin. Simply trim them with a knife. A tight roll is crucial, though, or the slices will unravel.