To make the sauce, combine together soy sauce, rice vinegar, sesame oil, water, and red pepper (chili) flakes. Season with salt and pepper. Set aside.
Heat oil in a frying pan over medium heat.
Add bok choy in batches, and cook for about 2 minutes on each side.
Remove from the pan.
To the same pan, add ginger and garlic. Stir fry for a few seconds.
Add bok choy.
Pour in sauce. Increase heat to high.
Toss them for a minute or two.
Sprinkle with sesame seeds.