First, make the gratin. Cut leeks into thin disks and soak in cold water for 20 minutes, to remove any soil between the layers. Drain into a colander.
Peel and very thinly slice potatoes – a mandolin is best for this. Now put a layer of one third of the potatoes into a greased oven-proof dish,
cover with a layer of half the sliced leeks. Season with a little black pepper. Top that with another layer of the next third of the potatoes,
then the remaining leeks, season again with black pepper, and finish off with the remaining potatoes. Pour over enough chicken stock to come half way
up the potatoes, season with more black pepper, dot with butter, cover tightly with kitchen foil and bake in a 170°C oven for 40 minutes. Then remove
the foil and continue cooking for another 15 minutes, until the top is browned and crisp. Let stand for 10 minutes before serving.
Meanwhile, wrap each haddock fillet in buttered kitchen foil, place on a baking sheet and bake in the oven for 20 minutes. To make the mustard sauce,
make a white sauce by melting a Tbs butter, stirring in a Tbs plain flour until well mixed, then stir in 200 ml whole milk, stirring continuously over heat
until smooth and thick, then add in 1-2 Tbs grain mustard, seasoning with a little salt and freshly ground black pepper. Serve poured over the haddock.