Add the salad potatoes to a large pan, leaving them whole. Fill the pan with water to cover the potatoes and season the water with a large pinch of salt. Bring the potatoes to a low boil and simmer for 20 mins till the potatoes are tender (a knife should easily pearce the skin).

While the potatoes cook, trim, peel and finely slice the red onion. Add the onion to a small bowl Halve the lemon and squeeze over the juice from half. Season with a pinch of salt and scrunch together. Set the onion aside to lightly pickle.

Add the mustard to a large salad bowl. Grate in the zest from the remaining half lemon and squeeze in the juice. Drizzle in 1 tbsp olive oil. Season with a pinch of salt and pepper. Whisk to make a smooth dressing.

Trim and discard the roots and any raggedy ends off the spring onions and finely slice them. Drain the olives and add them to the salad dressing. Slice the flat beans on the diagonal into 4-cm long pieces. Roughly chop the parsley leaves.

When the potatoes are tender, use a slotted spoon to remove them from the boiling water. Set them aside on a chopping board to cool slightly. Add the flat beans to to the boiling water and cook them for 3 mins till bright green and just tender. Drain the flat beans in a collander and rinse well under cold water.

Add the potatoes and flat beans to to the salad bowl. Spoon in the pickled red onions and toss well to mix.

Divide the salad between 2 plates. Scatter over the parsley leaves. Crumble over the feta and serve.