Heat the oven to 150°C/gas 2.

Peel the potatoes and slice them finely using a mandolin or a food processor. Place the potato slices in a large bowl of cold water and move them around to get rid of excess starch. Drain well and dry thoroughly: use a salad spinner or else put the slices in a tea towel, gather the corners together, go outside and swing your arm as fast and vigorously as you can ...

Rub a large shallow ovenproof dish with the garlic clove and a little butter.

Put the remaining butter with the cream in a large saucepan and bring just to the boil. Finely dice what's left of the garlic and add it to the butter and cream, along with seasoning and the potato slices. Gently simmer for 8 minutes.

Transfer to the prepared dish, spread out evenly and top with the Gruyere and a little more seasoning. Bake for 1 1/2 hours. Serve hot.