Sea Vegetable Gratin with Cheese

Slice the potatoes, celeriac, fennel, and courgette to 3mm on a mandoline or by hand. Slice the leeks into rounds. Mince the garlic. Grate all the cheeses, keeping the Parmesan separate. Rehydrate the wakame in cold water.

Drain the wakame thoroughly, squeeze out excess water, and roughly chop. Set aside alongside the prepared vegetables.

Heat the olive oil in a wide sauté pan over medium-high heat. Add the leeks and fennel with a pinch of salt and cook, stirring occasionally, until softened and lightly golden at the edges — about 8 minutes.

Add the garlic and courgette to the pan and cook, stirring, until the courgette is just tender and the garlic is fragrant — about 3 minutes. Remove from the heat and fold in the chopped wakame. Set aside.

Preheat the oven to 190°C (fan 170°C or 375°F). Butter a 28cm oval or 23 × 33cm rectangular gratin dish.

Melt the butter in a medium saucepan over medium heat. Stir in the kombu powder and cook for 30 seconds, then add the flour and whisk constantly to form a smooth roux — about 2 minutes, until it smells lightly biscuity.

Add the warm milk to the roux in a slow, steady stream, whisking vigorously to prevent lumps. Continue whisking over medium heat until the sauce thickens enough to coat the back of a spoon — about 5 minutes.

Reduce the heat to low. Stir in the double cream, Dijon mustard, nutmeg, white pepper, and three-quarters of the combined Comté and Gruyère. Stir until fully melted and smooth. Season with salt, taste, and adjust. Remove from the heat.

Arrange half the potato slices in an overlapping layer across the base of the gratin dish. Season lightly with salt. Spoon over half the leek, fennel, and courgette mixture, then scatter half the celeriac slices over the top. Pour half the béchamel evenly over this first layer.

Repeat with the remaining potato, vegetable mixture, and celeriac. Pour the remaining béchamel over the top, tilting the dish gently to ensure it flows through the layers.

Scatter the remaining Comté and Gruyère evenly over the surface. Combine the Parmesan and breadcrumbs and scatter over the cheese layer.

Cover the dish tightly with foil and bake for 35 minutes, until the potatoes and celeriac are tender when pierced with a skewer.

Remove the foil and bake uncovered for a further 20 minutes, until the top is deep golden brown and bubbling at the edges.

Remove from the oven and rest for 10 minutes before serving. Scatter the reserved fennel fronds over the top just before bringing to the table.