Beaten egg whites give this bake a light, soufflé-like texture, while gently softened leek and aged cheese bring a savoury centre. Serve it straight from the oven with a fresh green salad.
This bake gets its lightness from beaten egg whites, not cream or a thick sauce. The yolks bind the leek and cheese into a sturdy base, while the whites give the mixture volume. That makes it different from a regular frittata, which usually stays more compact. Beat the egg whites in a scrupulously clean, grease-free bowl until stiff peaks form. A little yolk in the whites can make them harder to beat, so separate the eggs carefully.
The leek is first gently softened in butter. This brings out its sweetness and mild flavour, without the fried, sometimes bitter taste of browned leek. Finely slice the light green part, too; very tough, dark green leaves are better saved for stock. Leeks are most widely available in the Netherlands during the cooler months, but you can make this dish year-round. Let the cooked leek cool before mixing it with the yolks: a hot filling can make the egg mixture set prematurely and makes it harder to fold in the whites without losing their airiness.
Aged cheese adds a strong, salty umami flavour and stands out alongside the soft leek. Grate the cheese yourself so it distributes evenly through the base. A mature Gouda works well; extra-mature cheese gives a slightly milder flavour. Stick to roughly the same amount, though, as much more cheese will make the batter heavy. A pinch of nutmeg complements the creamy, savoury flavours, but is used sparingly so the cheese still shines through.
Fold the egg whites into the base in two additions: stir the first portion in a little more firmly to loosen the batter, then gently fold in the rest using broad strokes. A few visible white streaks are better than beating out all the air. Use a baking dish that keeps the mixture from spreading too thinly, and try not to open the oven while it bakes; the heat helps the egg whites rise and set. The top should be golden brown, and the centre should feel just set but still springy.
Serve immediately, as the bake will gradually sink after baking; this is normal for a light egg dish. A salad of crisp leaves and cucumber with lemon adds freshness alongside the salty cheese. Once cooled, leftovers can be covered and stored in the fridge, then reheated thoroughly within two days. Bear in mind that the texture will be more compact than when the bake is fresh from the oven.
Preheat the oven to 190 °C (fan 175 °C). Lightly grease a round baking dish about 22 cm across with a little of the butter.
Melt the remaining butter in a frying pan over medium-low heat. Add the leek and cook gently for 8–10 minutes, until soft but not browned. Transfer to a plate and leave to cool.
In a large bowl, mix the egg yolks with the cooled leek, grated cheese, nutmeg, salt and a few turns of black pepper.
Beat the egg whites in a clean, grease-free bowl until stiff peaks form. Stir about a third of the egg whites into the leek mixture to loosen it.
Gently fold the remaining egg whites into the mixture in two additions. Spoon the batter into the baking dish and loosely smooth the top.
Bake in the centre of the oven for 22–26 minutes, until the top is golden brown and the centre feels just set and springy. Open the oven door as little as possible while it bakes.
Meanwhile, combine the salad leaves and cucumber. Whisk the lemon juice with the olive oil, a pinch of salt and some pepper, then toss the dressing with the salad just before serving.
Serve the bake immediately with the green salad.
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