Heat oil in a large nonstick skillet over medium-high heat.
Add beef; cook, stirring often, for 5 minutes or until crumbled and browned.
Add bell pepper and carrot; cook 1 minute.
Combine soy sauce, lime juice and brown sugar in a small bowl, stirring until sugar dissolves.
Stir into beef; cook 1 minute until heated.
Remove from heat, and transfer to an airtight container. Refrigerate meat mixture until chilled and ready to serve.
Stir in green onions, cilantro and mint.
Spoon heaping ½ cup beef mixture into center of each lettuce leaf.
