Halve the bok choy lengthwise and rinse it well under cold running water, making sure to gently separate the leaves to get any dirt out.
In a large saucepan, heat the oil over medium-high heat. Add the bok choy, cut side down, and sear for about 3 minutes without moving until the underside has started to turn golden brown.
In a small bowl, combine the broth, soy sauce, ginger, and garlic, and pour into the pan. Bring to a simmer over medium-high heat. Cover and simmer for about 6 minutes until the bok choy is tender but not mushy.
Remove the bok choy using tongs to a shallow serving bowl and return the pot with the braising liquid to medium-high heat. Simmer until the liquid is reduced to ⅓ to ½ cup, then add the sesame oil and pour the sauce over the bok choy on the platter. Garnish with scallions if desired.
Notes
Choose heads with full, dark leaves, and white stalks with no brown spots. Use as soon as possible and keep in the refrigerator, unwashed, in a plastic bag for no more than 5 days.
Before using, halve the bok choy lengthwise and rinse it well under cold running water, making sure to gently separate the leaves to get any dirt out.