Leek Gratin with Cheese

Wash the leek pieces thoroughly in a large bowl of cold water, lifting them out so grit sinks to the bottom. Pat dry with a clean towel. Measure and prepare all remaining ingredients — mince the garlic, grate all cheeses, and warm the milk in a small saucepan. Melt the 40g butter in a wide, heavy-bottomed sauté pan over medium-low heat, add the garlic, and stir for 30 seconds until fragrant but not coloured.

Add the leeks, salt, and white pepper. Stir to coat in the butter, then pour in the white wine. Cover tightly and cook over low heat for 30 minutes, stirring once halfway, until the leeks are completely tender and have given up most of their liquid.

Remove the lid and raise the heat to medium. Cook for a further 8–10 minutes, stirring occasionally, until nearly all of the liquid has evaporated and the leeks are jammy and beginning to caramelise at the edges.

Transfer to a buttered 28cm gratin dish and spread in an even layer.

Preheat the oven to 200°C, or 180°C for a fan oven, or gas mark 6. In a medium saucepan, melt 35g butter over medium heat. Add the flour and whisk constantly for 2 minutes until the roux smells faintly nutty and has turned pale gold.

Pour the warm milk into the roux in a slow, steady stream, whisking without stopping to prevent lumps. Cook over medium heat for 5–6 minutes, continuing to whisk, until the béchamel is thick enough to coat the back of a spoon.

Remove the pan from the heat. Stir in the Dijon mustard, nutmeg, Gruyère, and Comté until fully melted and smooth. Taste and adjust seasoning.

Pour the cheese béchamel evenly over the leeks. Scatter the topping Gruyère and Parmesan over the surface, then dot with the cubed butter.

Bake on the upper-middle rack for 22–25 minutes until the top is deeply golden and bubbling at the edges.

Rest for 5 minutes before serving.