Heat butter in a large saucepan over medium heat. Cook leek and garlic with a good pinch of salt for a few minutes or until soft.
Stir in the flour and cook, while stirring continuously, for about 1 minute.
Stir in about ½ cup of the milk, stirring continuously to avoid lumps forming. Gradually stir in the rest of the milk, then the stock and cream. Simmer for about 10 minutes, stirring often, until slightly thickened.
Add seafood (if using shellfish, add first and cook for 1-2 minutes before adding other seafood) and continue to simmer for a few minutes or until seafood is just cooked through. Season to taste with salt, pepper and lemon juice. Stir in parsley.
Ladle into bowls, distributing seafood equally, and garnish with chives.Serve with warm flatbread smeared with garlic and herb butter.