Preheat the oven to 375 degrees F. Place the fish in a single layer in a parchment paper-lined baking dish. Drizzle with the orange juice; season with salt and pepper. Bake 12 to 15 minutes, or until the fish is cooked through.
While the fish cooks, combine the stock, garlic, ginger, soy sauce, chilies and sugar in a small pot. Bring to a simmer, and simmer 2 to 3 minutes. Stir in miso until it dissolves.
Cover and reserve miso broth on low heat, do not boil. Steam the bok choy until just tender, about 1 minute. Divide and ladle the miso broth among 4 shallow bowls. Set a piece of fish, and 2 pieces of bok choy, in each bowl and serve.