On a cutting board, carefully cut off the excess stem ends at the base of each bunch of bok choy. Be careful not to cut too much of the stem end off or the stalks won't have anything to hold on to and they'll fall off from the bunch.
Cut the bok choy in half lengthwise. Once each bunch is cut in half, if you like, you can give the bok choy another rinse in between each stalk since they're more accessible.
Heat a skillet on low heat and add olive oil. With a heat resistant spatula, spread the olive oil around the pan (or carefully tilt the pan to spread the oil).
Add the garlic and ginger to the pan. Stir the garlic and ginger in with the oil, this not only coats the garlic and ginger but also adds flavor to the oil. Let cook (while stirring) until the garlic and ginger start to sizzle (about 3 minutes).
Then, lay the bok choy in the pan cut-side-down.
Pack the bok choy into the pan in a single layer. In a 12-inch skillet about 10 halves (5 small bok choy bunches) may fit in the pan if the bunches are on the small side.
Drizzle water over the bok choy and cover with a lid. Let steam (still on low heat) for 3 minutes or until the bok choy is just tender when pierced with the tip of a knife (but still has a slight crunch to it at the wide end of each stalk).
Drizzle soy sauce on top of the bok choy. Transfer the bok choy from the pan on to a serving dish.
Remove the pan from the heat and carefully transfer the remaining contents (bits of garlic and ginger, and liquid) from the pan into a small bowl. In the bowl, add sesame oil and stir.
Drizzle the sauce on top of the bok choy.
Serve warm as a side dish, either cut-side up or cut-side down. Garnish with sesame seeds.