Start by prepping your vegetables. Peel your celeriac, cut it in half, and chop it into thin slices. Halve your leeks, discarding the woody ends, and slice them into thin half-moons. Grate your garlic and pick your thyme leaves off their tough stalks.

Melt your butter in a large saucepan. Add your leeks and cook over a low-medium heat for 15 minutes until they are totally soft and sweet. Pop your garlic and thyme into the pan and cook out for 1 minute.

Pour your cream and milk into the pan and season with salt and pepper to taste. Add your celeriac and simmer for 15 minutes until your cream has thickened and your celeriac is just tender. Add your nutmeg.

Whilst your mixture simmers, grate your Gruyère. Combine with your panko breadcrumbs in a bowl and set aside.

Pour your celeriac mixture into a baking dish and top with your cheese and breadcrumbs. Cover with foil and bake in the oven for 20 minutes.

Remove your foil, then bake for another 20 minutes until golden and bubbly.

Serve up with some crusty bread, a roast chicken, or just on its own.