Lay the bok choy on a sheet pan, or two depending on their size, with the the cut side up.
In a mixing bowl combine the sesame oil, soy sauce, garlic, ginger, miso paste, and red pepper flakes if using. To make the mixture smoother use a blender and then spread over the bok choy using a pastry brush.
Roast for 5-10 minutes depending on the size of the bok choy. Remove from the oven before the leave start drying out.
While the bok choy is cooking toast the sesame seeds in a pan over medium high heat. Sprinkle over the cooked bok choy.