Rinse and dry the bok choy. Then, cut it in half lengthwise. The core can hide dirt, so you should rinse it thoroughly.
Heat the oil in a large 12-inch lidded skillet over medium-high heat. Add the garlic and cook, stirring, for 30 seconds.
Add the bok choy in a single layer, cut side down. Drizzle it with soy sauce and rice vinegar.
Cover the pan and cook until the core is fork-tender, 3-5 minutes. If the bottom of the pan becomes too dry, add a tablespoon or two of water.
Remove the pan from the heat. Arrange the bok choy on a serving plate. Drizzle it with the pan juices and sesame oil and sprinkle with red pepper flakes. Serve immediately.
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Notes
There's no need to add salt. The soy sauce adds plenty of saltiness. However, it's always a good idea to taste and decide for yourself. You can always sprinkle the finished dish with extra salt.
You can add grated fresh ginger in addition to the garlic. You can add both at the same time.
Instead of red pepper flakes, add a tablespoon of hot pepper sauce when you add the soy sauce and rice vinegar. Just remember that cooking pepper sauce can create fumes that some people are sensitive to.
The leftovers keep well in an airtight container in the fridge for 3-4 days. Gently reheat them in the microwave, covered, at 50% power.