Remove all the skin and any bones from the haddock. Put into a pan with the leek, milk and bouquet garni. Bring to a boil, cover and simmer gently for 8-10 minutes, until the fish flakes easily.
Strain, reserving the milk for making the sauce and discarding the bouquet.
Put the butter, flour and reserved milk into a pan. Bring to a boil and whisk constantly until smooth. Season, then add the fish and leek.
Cook the pasta in a large pan of boiling water until tender, but still firm to the bite. Drain and stir into the sauce with the chopped parsley. Serve immediately, garnishing with almond slivers if desired.