Scatter 1 tsp fine sea salt over eggplant and set aside to drain bitter juices (10 minutes). Rinse and pat dry. Heat 125ml of oil over high heat, then fry eggplant in batches, adding 100ml extra oil as oil absorbs (5-10 minutes). Drain on absorbent paper and set aside.
Add remaining oil to saucepan, then add onion and garlic and sauté over high heat until starting to colour (5-10 minutes). Add tomato and passata and bring to the boil, then season to taste and remove from heat.
Meanwhile, cook spaghetti in boiling salted water until al dente (8-10 minutes). Drain, then toss through tomato sauce and serve topped with eggplant, basil and ricotta salata.