Peel and finely chop the onion. Drain the tuna in a sieve. Wash the basil, shake dry and chop the leaves finely
Heat the oil in a pot. Sauté diced onions for 2-3 minutes. Add tomato paste and sweat it. Add tomatoes, season with salt, pepper, chilli and a little sugar. Pour on stock.
Cover and simmer for 10-15 minutes. Cook pasta in boiling salted water according to package instructions. After about 10 minutes add capers, tuna and basil to the sauce. Bring to the boil, season again and serve with pasta