Drizzle some olive oil in a pot or pan, then add the minced onion and turn on the heat.
When the onion starts to sweat, add the tuna together with its oil and the tomatoes.
Bring to a simmer, then turn the heat to low and let simmer for about 20 minutes. Season with salt, black pepper and chili pepper flakes to taste.If the sauce becomes too thick, add a few tablespoons of water.
Meanwhile bring a large pot of salted water to boil, then toss the pasta when the sauce is halwfay through the cooking.
Drain the pasta al dente and add it to the sauce. You can also drain it 1-2 minutes before it's done and let it finish cooking in the sauce.