Potato & smoked trout salad with mustard dressing

A light smoked fish salad with dill, capers and watercress in a tangy vinaigrette, use mackerel instead of trout if you like

800g potatoes, unpeeled
1 tbsp red wine vinegar
2 tsp dill, finely chopped
1 tbsp baby caper in brine, drained and rinsed
1 small red onion, thinly sliced into half moons
4 handfuls watercress, picked
175g smoked trout (or mackerel) fillets, skin and bones removed, flesh broken into chunks
2 tbsp wholegrain mustard
2 tbsp red wine vinegar
2 tbsp extra-virgin olive oil
1 tsp caster sugar

Put the potatoes in a saucepan of lightly salted water and bring to the boil. Simmer for 10 mins or until tender, then drain. When cool enough to handle, peel and cut into thick slices. Put in a bowl and toss with vinegar and some seasoning.
To make the dressing, mix all the ingredients together in a small bowl. Pour over the potatoes and add the dill, capers and onion. Arrange the salad on plates with a handful of watercress, then top with the smoked fish.