Remove the head from the fish (or have your fishmonger do this) and cut the fish into 2-inch thick steaks. Put the rice flour on a plate and season it well with salt and pepper; dredge the fish pieces in the rice flour, shaking off any excess.
In a deep pot or a wok large enough to hold the fish, heat the vegetable oil to 350 degrees F. Carefully put in the fish and deep-fry until it is browned, crisp, and cooked through, about 12 to 15 minutes. Remove the fish pieces and drain them on paper towels; place them onto a platter.
Carefully drain all but 3 tablespoons of oil from the wok. Stir-fry the scallions, ginger, garlic, and chiles for about 2 minutes. Add the black beans and fry for another minute. Pour in the chicken stock, bring to a boil, and simmer until the sauce begins to thicken, about 10 minutes. In a small bowl mix the cornstarch with 2 tablespoons water making sure to get out all the lumps. Pour this slurry into the black bean mixture (it will thicken immediately) and cook for another 2 minutes. Pour the sauce over the fish pieces, garnish with cilantro leaves, and serve.