Put onions, tomatoes and tuna in a bowl and pour sauce on top. Toss to coat evenly and refrigerate for at least 20 minutes.
Bring a large pot of water with salt (water should be salty to taste) and add spaghetti. Cook until al dente, according to directions on the package (about 6-7 minutes for thin spaghetti).
Rinse under cold water and drain well. Put spaghetti and sauce in a bowl and toss until coated evenly.
Season with salt and pepper and serve topped with Shiso leaves or basil and shredded Nori.