Put the fish bones in a pan with the bay leaf, parsley, and lovage. Cover with cold water, add salt and peppercorns, and bring to boil. Simmer for 30 minutes. Peel and chop the onion and carrot. Trim and chop the leek. Put them in a saucepan. Pour the fish stock over the vegetables through a seive (strainer). Bring to a boil, then let simmer for another 30 minutes, then put in the hake. Simmer for 15 minutes, or until the hake is cooked. Remove the hake and put the plaice (flounder) filets into the pan. Simmer for 5 minutes, then remove them. Cool the soup slightly, then puree it in a blender. Melt the butter in a clean pan, stir in the flour and cook for one minute. Stir in the blended soup and simmer for 3 minutes, stirring. Add salt and pepper to taste. Take the hake off the bone and chop it; skin the plaice (flounder) filets and chop them. Put both sorts of fish into the soup and stir in the sour cream. When all is hot, add the chopped fennel and stand, covered, for 3 to 4 minutes before serving. Serve as is, or with fennel dumplings.