Finely dice the peeled onions and garlic. Cut the chilli peppers lengthwise and remove the seeds. Cut them into narrow rings. Remove oregano leaves from stalks, finely chop half of them. Chop tinned tomatoes, keeping the juice. Heat the oil in a pan, add the onions, garlic, chillies, and chopped oregano. Sauté for 2 min. on medium heat. Stir in the olives and capers and sauté for 1 min. Add the tomatoes and their juice, simmer for approx. 15 min. Season the sauce with salt, pepper, and sugar.
Cook pasta until al dente. Drain the spaghetti and mix with the tomato sauce. Season with salt and pepper and serve sprinkled with freshly grated Parmesan and the remaining oregano leaves. Serve with chilli oil if desired.