Heat the oven to 200C/180C fan/gas 6 and drizzle 1 tsp oil over a non-stick baking tray. Press the aubergine into the oil, turn and bake for 10 mins. Mix the cheese with the hazelnuts. Turn the aubergine, top with the sage leaves and cheese mix and bake for 10-12 mins until tender.
Meanwhile, heat the remaining oil in a non-stick frying pan and fry the garlic, carrot, celery and chopped sage for 5 mins, stirring, until just soft. Add the olives, tomato purée, bouillon powder and 200ml water, cover and cook for 5 mins more.
Cook the spaghetti following pack instructions, then drain, reserving some of the cooking water. Toss with the cooked veg, tomatoes and most of the parsley, loosening with some of the reserved water if needed. Top with the aubergine and remaining parsley.