How to Make It
Process chiles, onion, vinegar, garlic, ginger, peppercorns, sugar, cumin, and salt in a blender until smooth, about 45 seconds. Reserve 2 tablespoons marinade for brushing.
Make 2 (6-inch-long) deep slits on each side of each fish parallel to the spine. Fill each slit with 1 teaspoon remaining marinade. Place 1 tablespoon flour in a fine wire-mesh sieve, and sprinkle evenly over both sides of 1 fish. Repeat procedure with remaining 3 fish and remaining 3 tablespoons flour.
Heat 2 tablespoons oil in a large nonstick skillet over medium. When oil is hot, add 2 fish. Cook, undisturbed, until skin is browned and crisp and fish is just cooked through, 5 to 7 minutes per side. Transfer to a platter, and loosely cover with aluminum foil to keep warm. Wipe skillet clean, and repeat procedure with remaining 2 tablespoons oil and remaining 2 fish. Brush fish evenly with reserved 2 tablespoons marinade, and sprinkle with salt to taste. Garnish with onion and cilantro. Serve with lime wedges and rice.