Put the oil and the onion, finely chopped, in a casserole and fry very gently. When the onion has become golden yellow, add the tomatoes, peeled, seeded and cut into pieces, douse with some ladlefuls of hot water, season with a pinch of salt and simmer for about 30 minutes. Break the spaghetti and add to the casserole. If the quantity of liquid is not enough to cook the pasta, add some ladlefuls of boiling water. In the meantime, beat the eggs and season with a pinch of salt, grated Parmesan and chopped basil leaves. When the pasta is cooked, pour the mixture made with the eggs into the casserole, give a good stir, and leave to gain a creamy consistency, away from the heat, without cooking. Distribute the soup in individual bowls and serve at table.