Boil the spaghetti in salted water, quench when you want it to be firm and set aside.
Peel the onion and the garlic cloves and chop them very finely. Sauté in hot oil. Chop the tuna and avocado, add to the onions and fry lightly for a few minutes. Finally, add the spaghetti to the pan and toss everything for a few minutes. Season with salt and herbs to taste.