Preheat oven to 400°F/200°C.

To a large pan, add the butter and oil over medium heat until melted and bubbly. Add the leeks and salt, cook, stirring often until slightly softened.

Sprinkle in the flour and stir to mix into the butter, cook for a minute to cook out the raw taste. Slowly drizzle in the milk while stirring or whisking. Add the mustard and nutmeg, stir until smooth and starting to thicken.

Add half of the cheese in small handfuls, stirring and waiting until each handful is melted before adding more. Once all the cheese is melted, stir until thickened. Taste for salt.

Pour into an 8 x 8-inch (20 x 20 cm) ovenproof baking dish and top with the remaining cheese and the breadcrumbs. Bake for 12-15 minutes until the top is golden brown. Serve with chopped parsley.