Preheat the oven to 240C. Line a baking tray with baking paper.
Place the potatoes in a large saucepan of salted water and bring to the boil over high heat. Boil for 15 minutes or until the potatoes are tender. Drain well.
Arrange potatoes on the lined baking tray. Using a potato masher, smash each potato so that it’s flattened but still holds together. Brush with the olive oil and season well with salt and pepper.
Bake the potatoes for 25 minutes or until crisp and golden. Cool slightly.
Meanwhile, mix garlic, parsley, thyme, basil and cheese together in a bowl. Place a spoonful of the herbed cheese on each potato, then finish with a slice of prosciutto piled on top. Serve the potatoes garnished with watercress.