In a small bowl, stir together the tamari, water, rice vinegar, mirin, sesame oil, garlic, and a pinch of red pepper flakes.

Heat the avocado oil in a large skillet over medium-high heat. Add half the bok choy, cut side down, and sear until browned on each side, 1 to 2 minutes per side. Remove, add more oil to the pan, if necessary, and add the remaining bok choy and repeat.

Add all of the bok choy back to the pan along with the sauce. Toss, then cover and cook 1 to 2 more minutes, or until the bok choy is tender.

Transfer to a platter, sprinkle with sesame seeds, and serve.