1. Heat a large skillet over medium-high heat. Add oil; swirl. Add ginger, mushrooms, and garlic; sauté 5 minutes, stirring frequently. Add stock and next 5 ingredients (through sugar); heat to 155°. Reduce heat to medium-low; arrange fish on top of mushroom mixture. Cover and simmer 10 minutes or until desired degree of doneness, maintaining a constant temperature of 150°.
2. Gently remove fish from pan; keep warm. Increase heat to medium-high; add onions. Cook, uncovered, 1 minute or until sauce is slightly thickened. Ladle 2/3 cup broth mixture into each of 4 shallow bowls; top each serving with 1 fillet. Sprinkle evenly with cilantro and pepper.