Peel and boil potatoes in usual way. Mash 4 potatoes with butter and beaten egg, and season to taste with salt and freshly ground black pepper. Force a border of mashed potatoes through a piping bag round a shallow ovenproof dish.
Sauté finely chopped onion in olive oil until transparent; dice remaining 2 cold boiled potatoes and add with diced lamb to onion; continue cooking until golden. Moisten with well-flavoured Tomato Sauce and simmer gently for 10 to 15 minutes, stirring from time to time.
Spoon hash into centre of potato border; sprinkle with freshly grated Parmesan and cook in a hot