Hearty Shiitake Mushroom and Miso Soup

INGREDIENTS

• 3 (6-inch) pieces dried kelp (kombu)
• 1/4 cup bonito flakes
• 3 ounces dried shiitake mushrooms
• 1 pound baby bok choy, cut in quarters
• 8 cups water
• 2 tablespoons toasted sesame oil
• 1 bunch scallions, sliced thin, white and green parts separated
• 1 (1-inch) piece fresh ginger, chopped
• 3 cloves garlic, chopped
• 1/2 cup light miso
• 8 ounces firm tofu, cut into cubes