Marinate the chicken thighs in sesame oil, soy sauce, cornstarch, and pepper. Allow the chicken to marinate for 20 - 30 minutes at room temperature.
Prepare the stir-fry sauce by combining chicken stock or water, soy sauce, sesame oil, rice vinegar, and sugar.
Heat cooking oil and sesame oil (1 tablespoon) in a wok over medium-high heat. Add the ginger and garlic to the wok while the oil is heating up so that the flavors are infused in the oil.
Saute the ginger and garlic until aromatic (about 1 minute).
Add the marinated chicken to the wok in a single layer. Leave the chicken untouched for about 30 seconds to allow it to sear before turning and stir-frying it.
When the chicken is fully cooked, remove the chicken from the wok.
Add baby bok choy and mushrooms to the wok. Stir fry for about 1 minute. Return the chicken to the wok.
Stir in the prepared sauce, then stir-fry to combine all ingredients.
Transfer the dish to a serving plate, then garnish with sesame seeds and chopped scallions. Serve with steamed rice topped with a fried egg and a splash of soy sauce.
Notes
You can substitute the baby bok choy with just about any vegetable you may have on hand. Some ideas: broccoli, zucchini, carrots, baby corn, asparagus, and green bell pepper.