Sliced potato, soft leek, cream and gruyere, baked until the top is amber and the inside is the kind of soft you can spoon. The side that pretends not to be the main event.
Why you should cook this
The first time I cooked a proper gratin I sliced the potatoes by hand because the mandoline had gone missing in a kitchen reshuffle the year before, and after twenty minutes of concentration I had a stack of potato discs in seven distinct thicknesses, ranging from cellophane to slate. The gratin came out unevenly cooked. Some discs had melted into the cream. Others had retained their structural integrity in protest. My wife ate two helpings, said “this is fine,” and went back to her email. That was the politest review of a dinner I have ever received.
That is the dish, executed properly. Sliced potato (uniform, two millimetres, mandoline non-negotiable), softened leek, layered with thickened cream and grated gruyère, baked at 180°C until the top is amber and the inside is the kind of soft you eat with a spoon. The technique is in the layering: thin layers of potato, a scatter of leek, a generous spoon of cream, a handful of cheese, repeat. Three layers minimum. The bottom layer goes in dry, on a buttered dish, so it crisps slightly into the base. The top layer gets the most cream and cheese so it forms the crust.
Use a waxy potato. Dutch creams are the conventional pick, kipflers if you can find them, desirees if your supermarket only stocks two varieties. Sebago will work but breaks down faster and the structure suffers. Definitely not floury potatoes; they collapse into the cream and you end up with a soup. The skin can stay on if the potatoes are scrubbed clean and unbruised; it adds a nutty flavour and saves time.
What to drink with it
This dish wants a Yarra Valley chardonnay with some restraint. The cream and the cheese want oak; the leek wants acidity. Yabby Lake Single Vineyard Chardonnay at $48 is the upgrade pour. The everyday bottle is a Mac Forbes RS3 at $30 from Dan Murphy’s; it has just enough oak to stand up to the cheese without smothering the leek. If white wine is not the move, a light pinot noir from the Mornington Peninsula (Ten Minutes by Tractor 10X at $35) works well, particularly if you are serving the gratin alongside a roast chicken.
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Notes from the kitchen
The leeks need a long, slow softening before they go anywhere near the gratin. Slice them thin (white and pale green only, the dark green tops go in the stockpot for another day), wash them thoroughly (leeks hide grit in their layers like a small geological sample), and sweat them in butter on the lowest heat for fifteen minutes until they have collapsed into a soft, sweet, almost-melted state. Raw leek in a gratin stays squeaky and slightly bitter. Soft leek tastes like a small private celebration.
The cream is straight thickened cream, not a béchamel. Some recipes call for a roux-based sauce; I find this overcomplicates a dish whose main pleasure is its lack of complication. Pour cream directly between the layers, season generously with salt and pepper at each layer (the potatoes are unsalted; they need every milligram of salt you put on them), and finish the top layer with extra cream and cheese. The cream will reduce in the oven and form a thick, glossy sauce that binds everything.
Two things that go wrong
Top is burnt and the inside is undercooked
Oven was too hot. Drop to 160°C, cover the top with foil, return to the oven for another 30 minutes. The inside should be tender to the point of a knife. The amber crust will hold; the foil protects it from going further.
Sauce is curdled or split
Cream got too hot too fast, or you used a low-fat cream. Use full-fat thickened cream (35%+ fat) and bake at a moderate temperature (180°C, not higher). If it splits during cooking, stir the surface gently with a spoon to bring the fat back together.
Variations worth knowing
Dauphinoise (the French classic)
Skip the leek. Rub the dish with a halved garlic clove before layering, and use only thinly sliced potato, cream and gruyère. A pinch of nutmeg between layers. The most elegant version, and the version your French mate will insist is the only legitimate one.
Smoked salmon and leek gratin
Same template, but layer in 200g of hot-smoked salmon flakes between the potato and leek. Skip the gruyère; finish with parmesan instead. The leftover-leek-from-Christmas dish that becomes a Boxing Day breakfast.
Sweet potato and leek gratin
Swap half the potatoes for sweet potato. Add a tablespoon of finely chopped fresh thyme between the layers. Finish with goat’s cheese instead of gruyère. Sweeter, earthier, the version that pairs with duck confit.
Leftovers and make ahead
Better the next day. The cream sets, the layers compact, the flavours integrate. Refrigerate covered for up to four days. Reheat in the oven at 160°C for 25 minutes with foil on; remove the foil for the last 10 minutes to crisp the top. Do not microwave. The microwave will give you a hot gratin that has lost its dignity.
Day-two gratin sliced into thick wedges and fried in a hot pan until the bottom crisps, then topped with a poached egg, becomes the best Sunday brunch dish in the cuisine. Eat at the kitchen counter, alone, while reading the paper. Tell nobody you only made the gratin yesterday.
The recipe
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Ingredients
Method
- Preheat oven to 180°C fan-forced. Generously butter a wide oval baking dish (about 30cm).
- Wash sliced leeks thoroughly. Melt butter in a wide pan over low heat, add leeks and a pinch of salt, cook gently for 15 minutes until soft and almost melted. Set aside.
- Slice potatoes uniformly to 2mm thickness using a mandoline. Do not rinse the slices; you want the starch.
- Whisk the cream with the grated garlic, nutmeg, thyme, generous salt and pepper.
- Layer half the potato slices in the dish, overlapping slightly. Season generously. Spread half the leek over. Pour over a third of the cream. Scatter a third of the cheese.
- Repeat with the remaining potato, leek, another third of the cream and a third of the cheese.
- Pour the remaining cream over the top, finish with the last third of the cheese.
- Bake uncovered for 50 to 60 minutes until the top is deep amber and the potatoes are tender to a knife. If browning too fast, cover with foil for the last 15 minutes.
- Rest 10 minutes before serving. The cream needs to settle into a glossy sauce.