Set a large, 4 to 6-quart stockpot filled with water over high heat and bring to a boil. Season the water with the kosher salt and add the buccatini. Cook the pasta for 5 minutes before starting the sauce. While the pasta cooks, set a 14-inch saute pan over medium-high heat and add the olive oil. Once the oil is hot, add the onions and garlic to the pan cook the onions, stirring occasionally until soft and slightly caramelized, about 3 to 4 minutes. Add the red pepper flakes and tomato paste to the pan. Drain the pasta from the pot and add it to the pan as well as the 1 1/2 cups of reserved pasta cooking water. Season the pasta with the salt and add the olives, tuna and parsley to the pan. Continue to cook the pasta, tossing to incorporate the ingredients, until most of the water has evaporated and the pasta is well coated with the sauce, about 3 to 4 minutes. Remove from the heat and serve with the Parmesan sprinkled on top.