For the quick pickles, combine rice vinegar, salt, sugar, cucumber and serrano in a bowl. Refrigerate at least 1 hour.
For the teriyaki sauce, combine sherry, tamari, pineapple juice, ginger, brown sugar, honey, hoisin and red pepper flakes in a saucepan. Bring to a boil, then simmer 15–20 minutes until reduced by half.
Mix cornstarch and water, stir into the sauce and boil 2–3 minutes until thickened. Remove ginger.
Preheat an indoor or outdoor grill to medium-high. Toss chicken with olive oil and season with salt and pepper.
Grill chicken 5–6 minutes per side. After flipping, glaze with teriyaki sauce; flip once more, glaze the other side and cook 1 minute longer. Rest 5 minutes.
Toast brioche buns. Build sandwiches with chicken, quick pickles and a dressing made from equal parts mayonnaise and teriyaki glaze plus a dash of sriracha. Drizzle with reserved teriyaki sauce.