Thoroughly wash and prepare the choy (see blog notes). Chop the stems into half inch slices and keep separate. Keep the leaves intact (unless large, then cut in half), and keep the center “tips” intact.
Put 2 Tb of cooking oil in a large saute pan over medium high heat, and place the ginger and garlic in the oil before the pan heats up (to keep the aromatics from burning). Saute them while the pan heats. When they have slightly softened, and before they take on any browning color, add the pepper flakes and stir a few seconds.
Add the stems of the bok choy to the pan and stir over the medium high heat for a couple of minutes. Add a few Tb of stock or water and allow it to cook off as the stems soften.
Add the choy leaves and tips together to the pan. Do this rather quickly in a few batches if necessary. (The leaves will cook down fairly rapidly so there will be room to add more.) Using tongs, stir and turn the choy leaves to cook for a minute. Add a few more Tb of stock or water along with the tamari, and stir well.
Continue tossing the choy as it cooks to the desired consistency of soft, bright green leaves and tender stems - neither should be mushy (should only take a few minutes), then turn off the heat. Drizzle the small amount of toasted sesame oil over the choy and stir very well. Serve with a sprinkle of toasted sesame seeds over each serving, if using.