Scrub the potatoes and cut them in half. Simmer in lightly salted water until just tender.
Meanwhile, place the mustard, lemon zest and juice, oil and salt and pepper in a jar, screw on the lid tightly and shake to combine.
Drain the cooked potatoes and toss with the dressing. Finely chop the spring onion, mint, capers and gherkins and add to the potatoes with the gherkin juice. Toss to mix evenly. Serve warm or cold.