Place potatoes in a saucepan of cold water and bring to the boil. Reduce heat to medium-low and simmer for 8 minutes until potatoes give slightly when pierced with a skewer. Drain and cool slightly, then place potatoes in a bowl with oil and sea salt, and toss well to combine. Slightly flatten each potato by pressing down with your hand and sit one potato on top of another, securing in place with a cocktail stick. Potatoes can be refrigerated at this stage for up to 24 hours.
Preheat oven to 190°C. Place potatoes on an oiled baking tray and bake for 10 minutes until crisp and golden. Remove cocktail sticks and replace with rosemary sprigs to serve.