Preheat oven to 200°C/400°F/Gas Mark 5. Heat olive oil in a frying pan and add onion, celery, carrot and garlic and fry until slightly golden, then stir in saffron and vinegar. Boil until the liquid has almost evaporated, then add wine and salt and bring back to the boil. Cut potatoes into ½ cm (¼ in) slices and place half in a 20 cm (8 in) square ceramic roasting dish. Sprinkle with parmesan and pour on half the onion mixture. Spread over remaining potatoes and pour on the rest of the onion mixture. Cover dish and bake until cooked, about 1 hour, remove lid and cook a further 10 minutes.