1. Prepare the sauce. Wash the leeks and cut in 1/2 inch pieces.
2. In a skillet melt butter and cook the onion until transparent. Add leeks and cook, stirring for 5 minutes. Add wine and tarragon. Cover and simmer 15 minutes. Uncover and cook until the wine has reduced to half. Add the cream and cook 10 minutes; add salt and pepper.
3. Let cool for 5 minutes. Place the mixture in a blender and blend until a smooth paste. Heat to serve with the fish.
4. Prepare the fish. Preheat broiler. Pat the fish dry. Brush each side with a little oil. Broil, turning once, until just opaque and lightly browned. Transfer to a plate and serve with leek or grelette sauce.