Preheat the oven to 180℃ fan/200℃/350F or Gas Mark 6.
Place the butter, double cream, milk, sliced garlic and thyme in a saucepan over a medium heat and stir every so often until the butter has melted then stir in the nutmeg and a good pinch of salt. Leave over a very low heat to keep warm.
Thinly slice the potatoes using a mandolin gadget or a sharp knife. You want them to be about 3-4mm thick.
Grease a baking dish with the extra butter then layer in 4 of the sliced potatoes. Pour over ¼ of the cream and garlic mix, then repeat these layers until you have used all of the sliced potatoes and all of the cream. (Discard the sprigs of thyme as you come across them when pouring in the cream.)
Cover the dish with foil and bake for 1 hour or until the middle of the dish is soft (test this by piercing it with a sharp knife and making sure there is no resistance.) If the potatoes aren’t quite cooked give them another 15 minutes then check again.
When the potatoes are cooked remove the foil, scatter over the grated gruyere and cook for another 15-20 minutes until the top is golden, or pop under a hot grill for a few minutes. Leave to stand for a few minutes before serving.
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Calories: 443kcal | Carbohydrates: 25g | Protein: 11g | Fat: 34g | Saturated Fat: 21g | Polyunsaturated Fat: 2g | Monounsaturated Fat: 9g | Trans Fat: 0.2g | Cholesterol: 106mg | Sodium: 209mg | Potassium: 651mg | Fiber: 3g | Sugar: 4g | Vitamin A: 1314IU | Vitamin C: 25mg | Calcium: 284mg | Iron: 1mg