Cook the potatoes for 20 minutes, or until fully cooked. Drain and puree, adding 45 ml (3 tbsp.) butter, 1 trickle of truffle oil, the milk or cream and the head of roasted garlic. Season and set aside.
While the potatoes are cooking, heat the duck legs confit in the oven or the duck meat confit in a frying pan.
In a large pan on low heat, lightly cook or sweat the leeks over low heat in 15 ml (2 tbsp.) butter until translucent. Season.
Preheat oven to 180°C (350°F).
Take an oven-safe dish and spread the shredded duck meat confit over the bottom, cover with the leeks and then with the potato puree, finishing with the grated Gruyere cheese. Bake for 20 minutes in the oven.