Heat olive oil in a saucepan over medium high heat. Add ground chicken and cook until browned, about 3-5 minutes, making sure to crumble the chicken as it cooks; drain excess fat.
Place ground chicken, bell peppers, onion, water chestnuts and rice into a 6-qt slow cooker. Stir in teriyaki sauce.
Cover and cook on low heat for 3-4 hours or high heat for 1-2 hours.
To serve, spoon several tablespoons of the chicken mixture into the center of a lettuce leaf, taco-style, garnished with green onions and sesame seeds, if desired.