Heat the sesame oil in a wok or large skillet or dutch ovenAdd the ginger and garlic and cook for a couple minutes, taking care the garlic doesn't burnAdd the tamari, then the bok choy, turning to coat in the oil and seasoning.Add ¼ cup of water and cover the pan with a lid. Steam the bok choy until the stem end is tender, about 4 or 5 minutes. Add more water if necessary.Make a slurry of the cornstarch with some water. Push the bok choy to the sides and pour the slurry into the center, stirring to make a sauce. It should thicken and become glossy.Remove the bok choy to a platter, pour the sauce over top and garnish with the sesame seeds.