1. Mix soy sauce, vinegar, brown sugar, sesame oil and five-spice powder in a bowl. Pour 1/4 cup of the soy mixture into a small saucepan and set aside. Stir 1/2 cup of the scallions and 3 tbsp of the ginger into remaining 1/4 cup soy mixture. Pour half over fillets in a resealable plastic bag. Marinate for 15 minutes. Toss bok choy with remaining marinade.
2. Meanwhile, bring 2 1/2 cups water to a boil in a small lidded pot. Add rice, cover and return to a boil. Reduce heat and cook 15 minutes, until water is absorbed. Keep covered, but remove from heat.
3. Fill a large skillet with 1/2 inch water. Stack 2 bamboo steamer baskets on top with lid. Bring water to a boil. Place fish fillets in bottom basket and bok choy in top basket. Cover with lid; steam for 6 to 8 minutes, until fish flakes easily with a fork.
4. While fish and bok choy are cooking, add stock and cornstarch to contents in saucepan; bring to a boil. Cook for 2 minutes, until thickened. Fluff rice. Serve fish and bok choy over rice, spoon sauce on top and garnish with remaining 1/2 cup scallions.