Method
Heat the grill to high.
Put the smoked haddock into a medium pan, you may need to cut it so that it fits in snugly. Pour over enough milk to just cover. Bring the liquid up to a gentle simmer and cook for 1–2 minutes, until the fish starts to flake easily to indicate it is just cooked, then turn off the heat. Remove the fish using a slotted spoon, and set aside. Reserve the milk. When the haddock is cool enough to handle, break it into large flakes, removing any bones or bits of skin as you go.
Melt the butter in a medium pan over a medium heat until it begins to bubble, then stir in the flour and cook gently for 2–3 minutes to cook out the flour. Gradually add about 150ml (5fl oz) of the reserved poaching milk, stirring all the time as you do so. When the sauce is smooth and thick, let it cook for 1–2 minutes, then add the Cheddar, the mustard and some salt and black pepper to taste. Remove from the heat and keep warm.
Bring a small pan of water to the boil. Add the chard leaves and cook for 2–3 minutes, until tender. Drain well, squeezing out the excess liquid with the back of a spoon, then fold the chard into the sauce along with the flaked fish.
Carefully spread the mixture over the toasted sourdough and place under the hot grill for 3–4 minutes, until bubbling and golden. Serve straight away.
