Preheat the oven to 350°F (180°C). You’ll need an ovenproof dish— I use an 8 ½-inch (22 cm) square baking dish, but you can size slightly up or down.

Heat a saucepan over medium heat and melt the butter. Gradually add the flour, a tablespoon at a time, whisking constantly to prevent burning or clumps. When well combined with the melted butter, add the hazelnut milk and 1 cup (240 ml) of water and whisk constantly for 2–3 minutes, until creamy and slightly thicker (you want the consistency of heavy cream). Whisk in the garlic, nutmeg, rosemary, soy sauce or tamari and season well with sea salt and black pepper. Remove from the heat and set aside.

Place the leek in the ovenproof dish, drizzle with some olive oil, season with sea salt and black pepper and toss. Press the leek into an even layer, then add ½ cup (120 ml) of the hazelnut cream and smooth it out with the back of a spoon.

Next, finely slice the potatoes using a mandoline or sharp knife (a mandoline makes this job so much easier and quicker). As you are slicing, try to keep the potatoes in a neat stack. Arrange the stacks of potato in the dish, edge-side up, until the dish is full. I usually like to place the stacks in an alternating crisscross pattern, but you can simply line them up across the dish or in a circular pattern—it’s up to you. The potato should be tightly packed.

Pour the remaining hazelnut cream over the potatoes, drizzle with olive oil and season with sea salt and black pepper. Place the dish on a sheet pan to catch any drips, then cover with foil and bake for 1 hour.

Meanwhile, place the sage leaves in a small bowl, drizzle with olive oil and season with a pinch of salt.

After 1 hour, remove the foil from the potato and scatter on the sage. Increase the oven temperature to 420°F (230°C) and bake for another 15–20 minutes, until the potatoes are golden and bubbling, and the sage leaves are crispy.

Serve with a green salad or steamed veggies.