Creamy Leek & Potato Bake with Gruyère

Begin by preheating your oven to 200°C (fan 180°C) and generously buttering a large, shallow baking dish that will accommodate all the ingredients in roughly three layers. The dish should be deep enough to hold the cream mixture but shallow enough to allow proper browning on top. Select the freshest leeks possible, looking for firm white bases and bright green tops with no yellowing or slimy areas.

Prepare the leeks with careful attention, as proper cleaning is essential for removing any grit trapped between their layers. Cut away the dark green tops and root ends, keeping only the white and pale green portions. Slice the leeks lengthwise, then cut into half-moons about 1cm thick. Place the sliced leeks in a large bowl of cold water and swish them around vigorously to release any trapped soil, then lift them out and rinse under running water until completely clean.

Peel the potatoes and slice them into rounds approximately 3mm thick – they should be thin enough to cook through properly but thick enough to maintain their structure during baking. A mandoline slicer is ideal for achieving uniform thickness, but a sharp knife and steady hand work just as well. Place the sliced potatoes in cold water to prevent browning while you prepare the leeks.

Heat the butter in a large frying pan over medium heat, allowing it to foam gently without browning. Add the cleaned leeks along with the minced garlic and cook slowly for 8-10 minutes, stirring occasionally, until the leeks become very soft and sweet-smelling. They should be completely tender and lightly golden but not browned. Season with salt and white pepper during cooking to build layers of flavor.

In a large measuring jug, combine the heavy cream, milk, fresh thyme leaves, and a generous pinch of freshly grated nutmeg. Whisk together thoroughly and season with salt and white pepper. The cream mixture should be well-seasoned as it will be the primary source of flavor for the potatoes during baking.

Drain the sliced potatoes thoroughly and pat them dry with clean kitchen towels. Begin layering the gratin by arranging one-third of the potato slices in overlapping rows across the bottom of the prepared baking dish. The slices should overlap like roof tiles, creating an attractive pattern while ensuring even cooking.

Spread half of the cooked leek mixture evenly over the first layer of potatoes, then sprinkle with one-third of the grated Gruyère. Create a second layer of potatoes, followed by the remaining leeks and another third of the cheese. Finish with a final layer of potatoes arranged in the most attractive overlapping pattern, as this will be the visible top of the finished dish.

Pour the seasoned cream mixture slowly and evenly over the assembled layers, allowing it to seep down between the potato slices and coat everything thoroughly. The liquid should come about three-quarters of the way up the sides of the dish – if needed, add a little more cream or milk to achieve the right level.

Sprinkle the remaining Gruyère cheese evenly over the top layer of potatoes, ensuring good coverage for proper browning. Cover the dish tightly with aluminum foil and bake for 35 minutes, during which time the potatoes will become tender and begin absorbing the cream.

Remove the foil and continue baking for another 15-20 minutes, until the top is beautifully golden brown and the cream is bubbling around the edges. The potatoes should be completely tender when pierced with a knife, and the surface should be attractively caramelized with some darker spots for extra flavor.

Allow the gratin to rest for at least 10 minutes before serving, as this resting time allows the cream to set slightly and makes serving much easier. The dish should be creamy but not soupy, with distinct layers that hold their shape when cut. This elegant side dish pairs beautifully with roasted meats, particularly lamb or beef, and also works wonderfully as a vegetarian main course alongside a crisp green salad and crusty bread.