For the tuna spaghetti, first put the spaghetti in boiling salted water and cook until al dente. Strain. Rinse with cold water and keep warm.
Heat the olive oil, fry the finely chopped onion until golden. Squeeze the garlic cloves and add them. Pour in tomato sauce. Mix in the chopped tuna, season with salt and pepper and simmer for about 5 minutes.
Arrange tuna spaghetti on plates.