Heat sesame oil in a medium skillet over medium heat. Add chicken and cook, stirring frequently, until lightly browned. Stir in soy sauce, vinegar, honey, ginger and lemongrass. Finely chop half the mango and add to the skillet. Cook for 5 minutes or until sauce is very thick; let cool. Spoon chicken into lettuce leaves and top each with remaining mango, bell pepper, green onion and cilantro.