In a large, deep skillet, bring broth, butter and garlic to a simmer. Add the bok choy and simmer (covered) until tender, about 5 minutes. Use tongs to transfer the bok choy to a serving dish and cover to keep warm.
Boil the broth mixture until reduced to ¼ cup, then stir in the sesame oil and red pepper flakes. Pour the mixture over the bok choy and serve immediately.
Notes
If preparing this recipe as gluten free, just be sure that you're using a brand of chicken broth that is known to be GF.