Drain the tuna. Cook pasta in 2-3 l boiling salted water (approx. 1 teaspoon salt per litre) according to package instructions.
Peel and finely chop the onion and garlic. Fry in hot oil. Add tomatoes, bring to the boil and simmer for about 5 minutes. Wash the basil leaves. Add to the sauce with tuna. Season with salt, pepper and 1 pinch of sugar.
Drain the pasta, mix with the sauce and serve.