Rinse the bok choy under cold water, making sure to remove all the sand and dirt from between the leaves. Cut the bunches in half.
Heat the butter in a sauté pan over medium-high heat. When it melts, add the bok choy, cut side down. Cook for 3-4 minutes until browned. Using tongs, turn the bok choy over, and garlic, ginger, soy, and stock. Cover and cook for another 2 minutes. Uncover and cook for another 1-2 minutes until most of the liquid has evaporated.