In a frying pan over a low heat, sweat the leek with a splash of olive oil for about 5 minutes until tender. Set aside to cool.
Boil the potatoes in a large pan of salted water for 15–20 minutes or until tender; drain. Return to the pan, and mash until smooth. Add the leeks, 1 egg, cheese, and parsley and mix together with a wooden spoon until it comes together. Season with salt and black pepper. Stir thoroughly. Chill for at least 1 hour to firm.
Spread the flour over a flat tray, and lightly beat the remaining 2 eggs in a shallow bowl. Put the breadcrumbs on a separate plate. Roll the chilled mixture into 12 balls of equal sized cylinders. Gently roll the croquettes in the flour, then dip each one in the beaten egg, and lastly coat in the breadcrumbs.
Heat the oil to 150°C in a large saucepan or deep fat fryer and deep fry for 5 minutes until golden brown. Cook in batches. Serve immediately.