Halve the onions and then cut into slices, finely chop the garlic. In a saucepan, fry the onions and garlic in olive oil. Pour off the oil from the tuna and put the tuna in the pan and heat briefly. Pour in the broth and bring to the boil. Immediately stir in the processed cheese and let it melt. Season with salt, pepper and sugar and thicken with a sauce thickener.
Pour the sauce over the pasta and serve immediately.