In a pan over medium heat, add the olive oil then the onions and garlic. Sauté until softened and fragrant, about 2 minutes.
Add the chili flakes, tomato sauce, and tuna (including the tuna olive oil from one of the tuna cans). Season with salt and pepper. Simmer for about 5 minutes to help marry the flavors.
In a large pot of salted, boiling water, cook the pasta according to package instructions, and drain.
Toss the pasta in the sauce, or plate the pasta on individual plate and spoon the sauce over the top. Garnish with fresh basil and additional red chili flakes if desired.