In a medium saucepan, heat oil over medium-high heat until smoking. Season the tuna with salt and pepper then sear in the oil until browned on all sides. When the tuna is browned, remove from the pan using a slotted spatula then set aside.
Discard excess oil then add 2 tablespoons of olive oil to the pot. Reduce the heat to medium, then add onions and garlic and sweat until translucent. When the vegetables are soft, add chili flakes and tomatoes, crushing the tomatoes as you add them to the pan. Lower the heat to medium-low and allow the sauce to come to a slow bubble. Add the tuna back to the sauce and cook slowly for approximately an hour, or until the tuna is tender.
Fill a large pot with water, bring to a boil and add salt. Drop the spaghetti into the water, stirring occasionally. As the pasta is cooking, gently reheat the tuna sauce, using pasta water as needed to adjust the consistency. When the past is al dente, remove from the water and add to the sauce. Add the pitted taggiasca olives and cook for 30 seconds mote. Finish with whole leave parsley and a splash of olive oil, then plate.