A gratin is what a potato was invented for. This one uses whole milk instead of cream, the starch from the potatoes thickens the milk into a glossy sauce as it bakes. Carrots add sweetness and colour. Cultured butter on top finishes the browning. ## The short version Slice potatoes and carrots thin. Layer in a buttered dish with salt and thyme. Pour warm whole milk over the top, dot with butter, bake until the top is deep gold. About an hour, almost all of it hands-off. ## What you need (Serves 4 to 6.) - 25 g cultured butter (for the dish and the top)

• 800 g organic potatoes, waxy varieties like Charlotte or Anya
• 300 g organic carrots
• 1 small clove garlic, halved (optional)
• 400 ml whole organic milk
• 1 tsp fine salt
• A few sprigs of fresh thyme
• Plenty of black pepper
• A grating of nutmeg ## How to make it 1. Preheat the oven to 180°C. Rub the inside of a baking dish (roughly 20 × 25 cm) with the cut side of the garlic, then butter the dish generously with about 10 g of the butter.

2. Slice. Peel the potatoes and carrots. Slice both as thinly as you can, 2 to 3 mm is ideal. A mandoline helps; a sharp knife is fine if you go slowly.
3. Warm the milk. In a small pan, warm the milk with the salt, a grating of nutmeg, and the thyme sprigs until it steams. Do not boil. Fish out the thyme sprigs (or leave them in, they shed their leaves in the dish).
4. Layer. Shingle the potato and carrot slices in the buttered dish, alternating as you go. Aim for slight overlaps, not stacks. Season between the layers with a pinch of salt and a grind of pepper. Fill the dish.
5. Pour and dot. Pour the warm milk evenly over the top. The milk should come just up to the top layer, it should not submerge the vegetables completely. Dot the remaining butter over the surface.
6. Bake. Bake for 55 to 70 minutes. The gratin is done when the top is deep gold, the edges are bubbling, and a knife slides through the centre without resistance.
7. Rest. Let it sit for 10 minutes before serving. The sauce needs this time to set. It will look more saucy hot and more layered warm. ## Why it works The starch on the sliced potatoes thickens the milk as it heats, this is why you slice them just before assembling and do not rinse them. The cultured butter on top carries the browning: the water evaporates, the milk solids toast, and the whole surface crisps into a sheet of gold. Waxy potatoes hold their shape; starchy ones dissolve into mush. ## Notes - Adding cheese? A handful of grated aged cheese over the top in the last 15 minutes is traditional in France, not Iceland. Try it without first.

• Make ahead. Fully bake, cool, refrigerate. Reheat covered at 160°C for 25 minutes, uncover for the last 5.
• Sliced too thick? Bake 15 minutes longer. Cover with foil if the top browns before the centre cooks.
• Pairs with. A green salad with sharp dressing, or, in Iceland, the Nordic sourdough rye from the first recipe and a glass of cold milk.