Boil the pasta following the instructions on the packet.
Poach the onion in the extra-virgin olive oil until golden brown. Take off the heat and put by.
Use the same oil to gently fry the garlic until golden brown, then add the tuna, tomato passata and the browned onion from earlier. Cover and cook for about 15 minutes over a low to medium heat.
When the spaghetti is al dente, add to the frying pan and cook for a few minutes to bind all the flavours together. Top off with some grated Parmesan and a pinch of fresh oregano.