Put 3 tablespoons of water in a large heatsafe glass bowl. Add the bok choy, ginger and garlic. Gently stir to combine. Cover with a vented lid or plastic wrap with 6 small holes poked in it. Microwave on high for 3 minutes. Carefully uncover and stir the bok choy. Cover again and cook on high until the bok choy is crisp-tender, about 3 more minutes.
Meanwhile, whisk together the miso paste, sesame oil, rice vinegar, 3 tablespoons water and a pinch of black pepper in a small bowl. Reserve.
Use a pair of tongs to transfer the bok choy to a serving platter, discarding any extra liquid and the garlic cloves left behind in the bowl. Drizzle with the miso sauce and garnish with the toasted sesame seeds.
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