1. Heat large nonstick skillet over high heat until hot, about 2 minutes. Add oil, swirl to coat pan bottom. Add bok choy stalks and cook, stirring frequently, until lightly browned, 5 to 7 minutes. Add garlic and cook, stirring frequently, until fragrant, about 30 seconds. Add bok choy greens, chicken stock, and salt and pepper to taste. Cover, reduce heat to medium-low, and cook, stirring twice, until bok choy is very tender, 8 to 10 minutes.
2. Remove cover, increase heat to medium-high, and cook until excess liquid evaporates, 2 to 3 minutes. (Bok choy should be moist but not soupy.) Stir in rice vinegar and adjust seasonings with salt and pepper; serve immediately.
STEP BY STEP: Preparing Bok Choy
1. Trim the bottom inch from the head of bok choy. Wash the leaves and pat them dry. With a chef's knife, cut the leafy green portion away from either side of the triangular white stalk.
2. Cut each white stalk in half lengthwise and then crosswise into 1/4-inch slices. Stack the leafy greens and then slice them crosswise into 1/2-inch strips. Keep the sliced stalks and leaves separate.