Preheat oven to 220°C (200°C fan forced). Line a large baking tray with baking paper.
Place the baby potatoes in a pot of salted water. Bring to a boil and cook until fork tender, about 15 minutes. Drain well.
Place the potatoes on the tray and gently press each one with the base of a glass until slightly flattened. Drizzle with olive oil and season with salt and pepper.
Roast for 40-45 minutes until the potatoes turn golden with crisp edges.
In a bowl, whisk together the mayonnaise, Greek yoghurt, Dijon mustard, vinegar, garlic, salt, and pepper.
Place the roasted potatoes in a large bowl (reserving one for garnish). Add red onion, spring onions, cucumber, capers and fresh herbs. Spoon over the dressing and gently fold to coat. Garnish with reserved crispy potato.
Serve warm, cold or at room temperature.