Preheat the oven to 180 degrees Celsius.

First cook the potatoes. Peel first if using older potatoes, but fresh dug or new potatoes will generally have thinner skins. Cover with boiling water and boil for about 10-12 minutes until the flesh is just beginning to yield when pierced with the tip of a knife. Drain in a colander and leave to cool slightly.

Melt the butter in a saucepan, add the flour and mix to create a roux (paste). Cook for one minute. Slowly pour in the milk while whisking constantly. Bring to a boil then turn down the heat and gently simmer for five minutes until thickened. Add the mustard, capers and juice, parsley, lemon juice and generous grind of black pepper. Cook for a further minute, then remove from the heat and add the chopped fish and drained potatoes. Tip into a shallow baking dish, approximately 20 x 25 cm.

To make the gratin, use a large knife to chop the rye slices into course bread crumbs (or use a food processor). Drizzle with melted butter and combine with the paprika. Spread evenly over the fish mixture. Bake for 30 to 35 minutes until the sauce is bubbling around the sides and the gratin is golden and crisp.

Serve with autumn salad leaves simply dressed with olive oil and a squeeze of lemon.