Leek, potato & three cheese gratin

Preheat the oven to 180ºC, gas mark 4 and lightly butter an ovenproof serving dish about 22x30cm. Trim the leeks and cut them into 1cm rounds. Wash well and drain.

Place the sliced potato into a large saucepan and cover with water, plus an extra 10cm or so. Bring to the boil, then add the leeks and return to the boil. Cover with a lid, simmer for 5 minutes, then drain.

Meanwhile, make the sauce. Melt the butter in a saucepan and whisk in the flour until smooth. Season. Add the milk gradually, whisking continuously. Cook for 1 minute, then add the blue cheese and grated hard cheese. Change to a heatproof spatula or wooden spoon. Cook over a low heat until the cheese is melted, stirring continuously.

Tip the drained potatoes and leeks into the buttered dish. Pour the sauce over the top. Mix the melted butter for the topping with the breadcrumbs and chopped nuts, then sprinkle over the top. Cover with foil and bake for 40 minutes. Remove the foil, sprinkle the grated Cheddar over the top and bake for another 20 minutes, until piping hot, golden and bubbling.