Combine soy sauce, brown sugar and fish sauce, set aside.
Heat 1 tablespoons oil in skillet over high heat. When hot, cook fish skin side down 2 minutes, or until lightly browned. Turn and cook one minute, then remove from skillet.
Reduce heat to medium and add remaining 1/2 Tbsp oil. Add onion, cook 2 minutes or until wilted and just beginning to brown. Add garlic, cook one minute. Add soy sauce mixture, stir well. Return fish to skillet and cook 2-3 minutes, or until sauce is dark and syrupy. Top with lime juice and black pepper.
Serve with steamed rice and vegetables (sauteed greens are good here!).