Season the chicken with the salt and pepper.
Toss the chicken with the 2 tablespoons of cornstarch until lightly coated.
Heat the vegetable oil in a large skillet over medium-high heat.
Sear the chicken for 2–3 minutes, just until lightly browned. It does not need to cook through.
Transfer the chicken to a 6-quart slow cooker.
In a medium bowl, whisk together the soy sauce, chicken broth, oyster sauce, hoisin sauce, rice vinegar, brown sugar, sesame oil, garlic, ginger, and crushed red pepper flakes, if using.
Pour the sauce over the chicken and stir to coat.
Add the mushrooms and zucchini.
Cover and cook on LOW for 4–5 hours, or until the chicken is cooked through and tender.
In a small bowl, whisk together the remaining cornstarch and cold water.
Stir the slurry into the slow cooker.
Cover and cook another 20–30 minutes, or until the sauce has thickened.
Sprinkle with sliced green onions and sesame seeds before serving.
Serve over steamed rice or chow mein noodles.