Place sesame seeds in a small dry skillet. Cook over medium-low heat, tossing, until golden, about 5 minutes. Transfer to a plate to cool.
In a large bowl, whisk together soy sauce, miso, lime juice, Worcestershire sauce, mayonnaise and sriracha. Stir in 1/2 of sesame seeds. Add chicken and toss to coat.
Divide chicken salad among 8 lettuce-leaf cups and place on a platter. Divide bell pepper and scallions among lettuce cups. Sprinkle each cup with remaining sesame seeds and herbs, if desired. Serve immediately.