Note: Mirin is a low-alcohol rice wine essential to Japanese cooking. Look for it in your supermarket with the Asian or gourmet ingredients. An equal portion of sherry or white wine with a pinch of sugar may be substituted.
Preheat oven to 450°F. Toss bok choy, oil, garlic and salt in a roasting pan. Roast on lowest rack, stirring twice, until wilted and tender-crisp, about 6 minutes. Whisk lemon zest and juice, tarragon, mirin and pepper in a small bowl. Drizzle over the roasted bok choy.