Equipment
* optional
Ingredients
Produce
- 1000 g starchy potatoes, peeled
- 300 g leeks, white and light green parts only
- 3 garlic, minced
Dairy
- 400 ml heavy cream, cold
- 200 ml whole milk, cold
- 150 g gruyère cheese, freshly grated
- 30 g unsalted butter, divided
Seasoning
- 10 g kosher salt
- 2 g black pepper, freshly ground
- 1 g nutmeg, freshly grated
- 2 g fresh thyme, leaves picked
Nutrition (per serving)
Method
Preheat oven to 190°C/375°F. Generously grease a 2-liter baking dish with 10g of butter.
Trim the leeks, slice them into thin half-moons, and wash thoroughly in a bowl of cold water to remove any hidden grit. Drain well and pat completely dry.
Melt the remaining 20g of butter in a sauté pan over medium heat. Add the leeks and a pinch of salt. Cook until softened and sweet, but not browned.
In a saucepan, combine the heavy cream, whole milk, minced garlic, kosher salt, black pepper, nutmeg, and thyme leaves. Bring to a gentle simmer over medium heat, then remove from the heat and let steep.
Slice the peeled potatoes into 3mm (1/8 inch) thick rounds using a mandoline slicer. Do not rinse the potatoes after slicing.
Arrange one third of the potato slices in the prepared baking dish in overlapping, shingled layers. Spread half of the sautéed leeks evenly over the potatoes. Repeat with another third of potatoes, the remaining leeks, and a final layer of potatoes.
Pour the warm cream mixture evenly over the layered potatoes and leeks. Gently press down on the top layer of potatoes to ensure they are fully moistened by the cream.
Cover the dish tightly with aluminum foil. Bake in the preheated oven at 190°C/375°F until the potatoes are mostly tender.
Remove the foil and sprinkle the grated Gruyère cheese evenly over the top. Return to the oven and bake uncovered until the top is deeply golden brown and a knife slides easily through the center potatoes.
Remove the gratin from the oven and let it rest at room temperature. This allows the potatoes to reabsorb the bubbling cream and ensures the gratin will slice cleanly rather than falling apart.
Chef's Notes
- Never rinse your potatoes after slicing them. The powdery starch left on the surface is the primary thickening agent for the sauce, naturally binding the cream and milk into a luxurious texture.
- If your hot cream mixture tastes slightly too salty on its own, it is perfectly seasoned. Potatoes absorb a massive amount of salt during the long baking process.
- For an extra layer of traditional flavor, cut an extra clove of raw garlic in half and rub the cut side vigorously around the interior of your baking dish before buttering it.
- Resting the gratin is non-negotiable. If you cut into it straight out of the oven, the hot cream will flood the empty spaces. Resting lets the starch set up and the liquids absorb.
Storage
Refrigerator: 4 days — Cover tightly with foil or store in an airtight container.
Freezer: 1 month — Potatoes may become slightly grainy upon thawing, but the flavor remains excellent.
Reheating: Cover with foil and reheat in a 160°C/320°F oven for 20-30 minutes until warmed through and bubbling at the edges.
