Step 1

Place potatoes in a large saucepan of salted cold water. Bring to the boil, then reduce heat and simmer about 15-20 minutes or until the potatoes are just tender. Drain well, transfer to a large bowl and sprinkle over the vinegar. Season with sea-salt and freshly ground black pepper.

Step 2

Meanwhile, to make mayonnaise; place the egg yolk and juice in a medium bowl; whisk until pale and creamy. Slowly add the oil, whisking constantly, until the mayonnaise is thick and creamy. Stir in mustard; season to taste with salt.

Step 3

Preheat oven grill to high and line a baking tray with baking paper. Heat a frying pan over a medium heat and add the olive oil and onions; cook for 15-20 minutes, turning occasionally until golden brown colour.

Step 4

Place prosciutto slices in a single layer on the lined tray and grill until golden brown and crispy, turning over half way through.

Step 5

Add half the mayonnaise, half the herbs and half the onions; toss gently to combine and transfer to a serving platter. Drizzle remaining mayonnaise over potatoes, top with remaining herbs, onions and roughly broken pieces of prosciutto.