Cook 750 g small potatoes in salted boiling water until tender, drain and cool slightly, then cut into bite-size pieces and tip into a large bowl.
Trim and finely slice 1 bunch of spring onions.
Cut 6 radishes into fine matchsticks.
Roughly chop 6 cornichons and 1 tablespoon capers.
In a small bowl whisk together 2 tablespoons wholegrain mustard, 1 tablespoon white wine vinegar and 3 tablespoons olive oil and season with salt and freshly ground black pepper.
Pour the dressing over the potatoes, add the spring onions, radishes, cornichons, capers, 2 tablespoons roughly chopped chives and 2 tablespoons freshly chopped flatleaf parsley and gently mix together.
Serve at room temperature.