Drain the tuna and let it drain well. Peel and halve the onions and cut them into thin strips. Peel garlic and chop finely. Wash the basil and remove the leaves.
In a deep frying pan with lid or a wide pot, mix tomatoes, tomato paste, 3⁄4 l water, garlic, chili flakes, 1 1⁄2 tsp salt and 1 tsp pepper. Pluck the tuna. Add capers, onions and basil to the tomato sauce.
If necessary, break through the spaghetti once and put it into the pot. Cover with the sauce. Cover and bring to the boil over medium heat, simmer for about 15 minutes, stirring 3-4 times. Season to taste and serve.