Marinade: in a gallon-sized sealable plastic bag, combine broth, soy sauce, Worcestershire, brown sugar, cornstarch, sesame oil and baking soda, and add chicken. Seal bag with no air, turn to coat chicken. Refrigerate, turning bag occasionally, at least 2 hours or overnight. Drain and discard marinade. Heat skillet, add sesame seeds and stir-fry until golden, 2-3 minutes. Transfer to small bowl. In skillet, heat peanut oil. Stir-fry scallions, garlic, and pepper flakes 2 minutes; add asparagus and stir-fry until tender, about 4-5 minutes. Add chicken and watercress, stir-fry until chicken is cooked through, 6-8 minutes. Serve, sprinkled with sesame seeds.