1. Pre-heat the oven to 160°C. Peel the potatoes and place them whole into cold water.
2. Pour the cream into a pan, and then add the garlic, rosemary and the onion. Heat them to a boiling point and simmer for 5 minutes.
3. Allow them to cool slightly, then strain into a jug or another pan. Slice the potato directly into the warm, flavoured cream. Lightly cover your dish with a thin layer of oil. Once the potatoes and cream are well mixed, begin to layer the potato into your dish, adding some Gruyere cheese and seasoning between each layer.
4. Once the dish is full and you’ve used all the cheese. Pour a little cream over the top to help the cooking process.
5. Bake in a bain marie by placing the dish into a larger dish and filling halfway with hot water, then put it into the oven for 1 – 1 ½ hours. Once cooked, rest it for 10 minutes and then serve it.