1. Peel the potatoes. Use a knife to round off the edges to form a cylinder shape and then cut each cylinder into 3-4 mm thick slices. Do not wash slices.
2. Rub a large saucepan with garlic then butter.
3. Add salt, pepper, the potato slices and the cream and milk. Cook over low heat.
4. When the potatoes are cooked, remove with a slotted spoon to a butter gratin dish.
5. Strain the potatoes cooking liquid through a chinois directly onto the potatoes.
6. Sprinkle with grated Gruyere. Place the gratin dish in a bain-marie and bake at 355 degrees F (180 degrees C) until the top is brown. Serve gratin in its dish.