Preparing the sauce: grate the onion and the garlic, and brown them in olive oil.
Add the tomato juice, salt, pepper, oregano, and a little water.
Bring to the boil and let the sauce cook for 20 minutes stirring occasionally.
When the sauce is ready and just before removing it from the heat, cut the tuna in chunks, strain the oil and add it to the sauce.
The sauce is ready to be served with the spaghetti.
Grated cheese can be added.