Cut flounder into fillets, reserving bones. Remove skin from fillets, and discard. Cut fillets into 2-inch pieces, and cover and chill.
Combine fish bones, water, peppercorns, dill, and 1 cup parsley in a large Dutch oven; bring to a boil. Cover, reduce heat, and simmer 30 minutes. Drain mixture in a sieve over a bowl, reserving stock. Discard solids.
Heat oil in pan over medium heat. Add sliced leek and onion; sauté 10 minutes or until tender. Sprinkle flour over leek mixture; cook 1 minute, stirring constantly. Gradually add reserved stock, stirring until well-blended. Add potatoes and next 5 ingredients (potatoes through bay leaves); cook 25 minutes or until vegetables are tender. Add fish, wine, 1/4 cup parsley, vinegar, and mustard; cook over low heat 5 minutes or until fish flakes easily when tested with a fork. Discard bay leaves. Ladle soup into bowls; top with shrimp,and garnish with slivered leek, if desired.
Note: Juniper berries can be found in the spice section of most supermarkets. Your fish monger will be happy to fillet the flounder for you.