375g/12oz fingerling potatoes
1 tablespoon mustard oil
1 teaspoon Dijon mustard
1 teaspoon rice vinegar
1/2 teaspoon wasabi powder
1/2 teaspoon kosher salt
4 whole pickled-herring fillets
Put the potatoes in a medium saucepan, add salted water to cover by 2cm (1in) and bring to the boil. Cook for 20 minutes. Drain.
When cool, peel the potatoes and mash well with a fork. Add the mustard oil, Dijon mustard, vinegar, wasabi and salt. Mix well.
Divide the potato mixture into four. Roll each portion under your palms into a log about 2cm (1in) in diameter and 20cm (9in) in length.
Slice the herring on the diagonal into 1cm (1/2in) wide strips. Cut the potato logs into 3cm (11/2in) lengths and top each one with a piece of herring. Serve chilled.