Boil the potatoes for 15 min. in boiling water with plenty of salt. Leave them to drain well and cool them down slightly.
Preheat the oven to 240˚C (460˚F). Divide the potatoes over a lightly oiled baking tray or cover with baking paper. Press each potato flat with a spoon or other tool. Sprinkle with olive oil and add salt and pepper.
Roast the potatoes for about 20-30 minutes or until crispy and golden brown. Leave to cool for about 10-15 minutes.
Whisk the garlic, herbs and creme fraiche (or ricotta or creamcheese) together in a bowl and put a spoonfull on top of each potato. Put a slice of prosiutto on top and serve the potatoes on a bit of watercress.