Smoked trout with potato salad and mustard oil

Main course: serves 4-6. Total time: 30 minutes plus 24 hours infusion

Mustard dressing for the potato salad is optional - you can use mayonnaise, creamed horseradish and a dot of mustard instead.

225-350g/8-12oz smoked trout fillets
For the mustard oil
50g/2oz dried mustard seeds
1 tablespoon water
1 teaspoon English mustard powder
5 tablespoons grapeseed oil
1 teaspoon ground turmeric
A pinch of salt
For the potato salad
450g/1lb cooked warm, small new potatoes
1 teaspoon finely grated horseradish or horseradish sauce
1 red onion, finely chopped
1 tablespoon olive oil
A squeeze of fresh lemon juice
Salt
Freshly ground black pepper
4-6 tablespoons mustard dressing (see below) or mayonnaise
1 teaspoon chopped fresh parsley
Salad leaves, to serve (optional)

Mustard dressing

To make the mustard oil, roast the mustard seeds in a dry frying-pan over a moderate heat for a few minutes until golden. Allow to cool. Mix the water with the mustard powder, then blend all the ingredients together. Leave the mix for 24 hours to infuse the flavours. Then pass through a muslin cloth or very fine sieve to achieve a clear mustard oil.

Now make the potato salad. As soon as the potatoes are cooked, cut them in half and add the red onion, olive oil, lemon juice and season with salt and pepper. To finish simply add the mustard dressing, or mayonnaise and chopped parsley. To serve the dish, sit the potato salad on plates and top with the smoked fish, or serve at the side. Finish the dish with a trickle of the mustard oil. Salad leaves can also be used, dressed with a little mustard oil.

Mustard dressing: Whisk the mustard with the vinegar. Mix together the two oils and gradually add to the mustard, whisking all the time, as you would if making mayonnaise. Add salt and pepper to taste. For a creamier texture, beat in one or two egg yolks with the mustard and wine