Heat butter in a large heavy pot over medium heat. When butter is hot, add the leeks and garlic and sauté, stirring until softened, 3 to 4 minutes. Sprinkle the flour over the leek mixture and stir constantly for 2 minutes. Add the milk and stock and stir until mixture is smooth, slightly thickened and flour is well dissolved, about 3 minutes. Add the tomatoes, potatoes, tomato paste, bay leaves, cayenne and salt. Stir well to mix. Bring soup to a simmer and cook, stirring occasionally, until potatoes are tender, about 25 minutes. (The soup can be made a day ahead to this point. Remove from heat and cool. Remove bay leaves. Then cover and refrigerate.) To finish soup, reheat over medium heat, stirring until simmering. Add scallops and cook, stirring until scallops are opaque and cooked through, about 4 minutes. Taste and add more salt, if desired, and black pepper to taste. Ladle into 6 individual soup bowls and sprinkle with parsley.