Preheat the oven to 425 degrees.
Cut the bok choy in half lengthwise, and arrange it on a large baking sheet. (If you are using the large heads of bok choy, cut it into quarters lengthwise.)
In a small bowl, whisk together the olive oil, sesame oil, tamari, garlic, and red pepper flakes.
Spoon the marinade over the bok choy, and then use your fingers to gently massage the bok choy so that you get it into all of the layers.
Roast for 10-12 minutes, until lightly wilted and tender-crisp. When serving, sprinkle the sesame seeds over the top for garnish.