Wash and peel the potatoes and, with a potato peeler, scoop out the centres of the potatoes, leaving a shell about a quarter-inch thick. Put the potato shells in cold salted water and put the centres into a little water to cook until they are tender. Drain and purée.
Meanwhile, in a non-stick frying pan, sauté the mince until browned and then mix with the potato purée, breadcrumbs, cheese, seasoning, garlic and parsley and beaten egg. Stuff this mixture into the potato shells. Heat the oil and sauté the onion and celery. Add the potatoes and cook them carefully until the shells are golden.
Remove the potatoes, then add the tomatoes and sugar. Stir to amalgamate and then put back the potatoes, cover and cook gently for half an hour or so, until the potatoes are tender when tested with a skewer. Serve with the tomato, onion and celery on the side.