1. Combine first 3 ingredients in a medium bowl, mixing gently until just combined; let stand at room temperature 15 minutes.
2. Heat a large nonstick skillet over medium-high heat. Add pork mixture to pan; sauté 5 minutes or until pork is browned, stirring to crumble. Place pork mixture in a large bowl. Add cucumber and next 5 ingredients (through serrano), and stir to combine.
3. Combine lime juice, fish sauce, and sugar in a small bowl, stirring until sugar dissolves. Add juice mixture to pork mixture; toss. Place about 1/3 cup pork mixture in each lettuce leaf.