Cook potatoes in salted boiling water until tender. Drain and cool slightly. Peel and mash with milk and butter. Set aside. Cook leeks and onion in a small quantity of salted boiling water for about ten minutes or till tender. Drain.
In a large bowl, combine leeks and onion with mashed potatoes, cheese, parsley, salt and black pepper powder. In another bowl, beat together egg and two tablespoons water. Divide the potato mixture into equal portions and shape into round cakes.
Brush them with the egg mixture. Coat them well with the breadcrumbs. Heat sufficient oil in a non-stick pan. Place potato cakes in batches and fry on low heat for two to three minutes or until light golden brown from both sides.
Drain on absorbent paper. Garnish with parsley sprigs and serve hot with mixed salad greens.