Heat oven to 175 degrees. Place chicken stock in a wok or skillet, and bring to a simmer. Cut fish into pieces about 3 inches square, place in stock, and simmer until just cooked, about 5 minutes. Use spatula to transfer to heatproof platter, cover loosely with foil, and place in oven. Drain and strain stock, and reserve. Wipe out pan.
Heat peanut oil in pan. Add garlic, ginger and scallions. Sauté briefly, and add mushrooms. Sauté until wilted. Add soy sauce, vinegar and fish sauce. Cook 30 seconds, then add reserved stock. Bring to a simmer. Dissolve cornstarch in 2 tablespoons cold water, and add, stirring. Simmer until sauce has thickened. Add sesame oil.
Transfer fish to serving dish, spoon mushroom sauce over, scatter with cilantro, and serve.