Cook peeled potatoes. Drain water and purée potatoes in a food processor or through a sieve. Add all other dough ingredients and mix well.
Grease and dust with flour a rectangular baking pan (25cm x 35 cm / 10 in x 14 in). Pour in mixed dough, spreading it evenly and bring it to a pre-heated oven (200 C/375 F/British Gas Mark 5) for 30 minutes.
Filling: Mince onions, green pepper and garlic. Fry on hot vegetable oil adding the ground beef, salt and pepper. Fry well allowing most of the liquid to evaporate.
Place the still hot baked dough on a clean cloth hand towel (on a dry, flat surface), spread filling evenly on top of baked dough and starting from one edge, begin to roll it up until it turns in the shape of a cylinder. Now grab the "cylinder" with the help of the towel still underneath it and carefully turn it over onto a large serving plate and remove towel very slowly in order to avoid breakups.
Top with hot tomato sauce, sprinkle the parmesan cheese and garnish with olives.
Serve with spaghetti and a hearty vegetable salad.
Variations: Substitute 500 g (1 lb) of fresh mushrooms and 3 sliced onions for the ground beef. Fry on hot vegetable oil until liquid evaporates and season with salt and pepper. Try also a different filling with your preferred shrimp pâté or simply with 1 kg (2.2 lb) of fried spinach seasoned with salt.