Wash and clean duck. Rub the inside with Vegeta and the outside with salt. Place in a roasting pan, pour hot oil over duck, cover with aluminium foil and bake at 220°C about 1 hour.
Meanwhile, make dumplings. Mix flour, egg, salt and water to form a firm dough. Shape dumplings and boil in salted water about 5 minutes.
Remove roasting pan with duck, drain off part of drippings and add drained pelati tomatoes, olives, crushed garlic and herbs. Add sherry and wine, and place butter slices on duck. Continue baking without foil at 200°C for 15 minutes. Reduce temperature to 180°C and roast for another hour (if necessary, reduce temperature to 150°C). While baking, baste duck with warm water.
Remove roasted duck. Thicken vegetable gravy adding cornstarch mixed in a little water.
Carve roasted duck and pour a little olive oil over the pieces. Serve with vegetables gravy and dumplings.
If desired, serve duck with a different side dish, such as palenta, roasted potatoes or gnocchi.