- Prep
- 25 min
- Cook
- 75 min
- Total
- 100 min
- Serves
- 6
Ingredients
What you’ll need — tap to tick off as you go
- 1200 g
- 30 g
- 2 cloves
- 300 ml
- 200 ml
- 4 g
- 1 g
- ½ g
- 150 g
Process
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Preheat the oven to 180°C. Grease a 23 by 33 cm baking dish with a little unsalted butter.
Melt the 30 g unsalted butter in a large skillet over medium heat. Add the sliced leeks and cook for 8 to 10 minutes, stirring often, until soft and sweet but not browned. Stir in the minced garlic and cook for 1 minute more.
In a saucepan, warm the heavy cream and whole milk with the salt, black pepper, nutmeg and thyme sprigs over low heat until steaming. Do not boil. Remove the thyme sprigs.
Layer half of the sliced potatoes in the prepared dish, then spread all of the cooked leeks over them. Top with the remaining potatoes, arranging the slices in overlapping shingles.
Pour the warm cream mixture evenly over the potatoes. Press the layers down gently with a spatula so the liquid settles between the slices.
Cover the dish tightly with foil and bake for 45 minutes. Remove the foil, scatter the grated Gruyere cheese over the top and bake uncovered for 25 to 30 minutes more, until the cheese is golden and bubbling and a knife slides easily through the potatoes.
Let the gratin rest for 10 to 15 minutes before serving so the layers set. Garnish with fresh thyme leaves if desired.
Kitchen notes
Kitchen notes
- The most common mistake is slicing the potatoes too thick. Use a mandoline or food processor on the 3 mm setting so every slice cooks in the same time; uneven slices mean crunchy centers under a burnt top.
- Wash the leeks thoroughly after slicing: swirl them in a bowl of cold water, then lift them out so the grit sinks. Sand hidden between the layers is the classic leek failure.
- Warming the cream and milk before pouring it over the potatoes helps the starch start softening immediately and shortens the bake. Cold liquid can leave the center undercooked.
- The gratin is done when a paring knife meets no resistance in the center and the liquid around the edges has thickened to a creamy sauce, not a watery pool.
Substitutions
- Gruyere cheese→Comte, Emmental or a sharp white cheddar
- heavy cream→half-and-half or whole milk with 1 tbsp extra butter
- leeks→2 medium onions, thinly sliced and cooked the same way
- fresh thyme→1 tsp dried thyme
Storage & reheating
Cool leftovers to room temperature, then store in an airtight container in the fridge for up to 4 days. To reheat, cover with foil and warm in a 180°C oven for 15 to 20 minutes, or microwave single portions for 2 to 3 minutes until steaming. The gratin can be assembled up to 24 hours ahead, covered and refrigerated; add 10 minutes to the covered baking time. It does not freeze well because the cream sauce separates and the potatoes turn grainy.
FAQ
The questions we get asked most
Why is my potato leek gratin watery?
The potatoes released too much liquid, usually because they were sliced too thick or the dish was underbaked. Bake it uncovered for another 10 to 15 minutes at 180°C until the cream around the edges bubbles and thickens. Next time, slice the potatoes 3 mm thick and let the finished gratin rest 15 minutes before serving so the sauce reabsorbs.
Can I make potato leek gratin ahead of time?
Yes. Assemble the gratin completely, cover and refrigerate for up to 24 hours. Bake covered at 180°C for 55 minutes instead of 45, then uncover, add the cheese and bake 25 to 30 minutes more. If you have already baked it, reheat covered with foil at 180°C for 20 minutes.
Can I use milk instead of cream?
Yes, but the result will be less rich and more likely to curdle. Use 500 ml whole milk with 30 g extra butter melted in, and keep the oven at 180°C. Do not use skim milk, as the sauce will look thin and split.
How do I know when the potatoes are fully cooked?
Insert a paring knife into the center of the gratin; it should slide in with no resistance at all. The top should be deep golden brown and the cream should be bubbling thickly around the edges. If the knife meets any firmness, cover with foil and bake 10 more minutes.
Can I freeze leftover potato leek gratin?
Freezing is not recommended. The cream sauce separates on thawing and the potato slices become watery and grainy. If you must freeze it, do so in single portions for up to 1 month and reheat from frozen, covered, at 180°C for 30 minutes, though the texture will be softer.
Why did my leeks turn brown and bitter?
The heat was too high or the leeks were not stirred often enough. Cook them over medium heat for 8 to 10 minutes until soft and translucent, not browned. If they start to color, lower the heat and add a splash of water to keep them sweet.
Can I add other ingredients like bacon or mushrooms?
Mushrooms work well: saute 200 g sliced mushrooms with the leeks until their liquid evaporates. For a halal version, skip bacon and use thinly sliced smoked turkey or extra mushrooms for a savory note. Add any extras between the potato layers so the gratin stays structured.
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