Grated carrot supplies an interesting bit of color.
Combine spinach and ½ cup stock in saucepan. Bring to a boil, reduce heat, and simmer, uncovered, 5 minutes. Press through food mill and set aside. Melt 2 tablespoons butter in soup kettle, saute onions and carrot until tender. Sprinkle with flour and blend with whisk. Slowly add remaining 4-½ cups stock and stir until smooth. Add pureed spinach, bouquet garni, and peppercorns. Season with salt and pepper to taste. Bring just to a boil, reduce heat, and simmer, partially covered, 10 minutes. Heat remaining 2 tablespoons butter in skillet. Add wine, bring mixture to a boil, reduce heat. Add flounder, cover, and simmer 5 minutes. Spoon flounder into soup bowls and cover with hot soup.