Preheat broiler. In a food processor or blender, combine the cilantro, garlic, lime zest and juice, jalapeño, and soy sauce. Blend until mixed. With processor or blender running, gradually pour in the oil and process until thickened.
Place the fish steaks on a greased broiler pan. Brush top of each steak with the sauce. Coat the potatoes with remaining sauce; place in broiler pan with the fish.
Broil about 10 minutes, or until fish flakes with a fork.