Step 1Heat 2 tablespoons of the olive oil in a large skillet over medium heat. Add the onion and cook until it's transparent, about 3 to 5 minutes. Add the garlic, 2 tablespoons of chopped parsley and the chile, and cook for 1 minute. Add the canned tomatoes with their juices to the pan, and cook the sauce uncovered, stirring occasionally, until the sauce is thick, about 15 minutes. Once the sauce has thickened, add the tuna and olives, stir gently to break up the tuna, and cook until the tuna is warmed through, about 2 minutes.
Step 2Meanwhile, cook the spaghetti in well-salted boiling water until al dente, slightly firm to the bite.
Step 3While the sauce is simmering, warm the remaining 2 tablespoons of oil in a small pot. Add the anchovies, remove the pot from the heat, and, using a fork, mash the anchovies into the olive oil until they dissolve. Stir in the capers and the remaining 2 tablespoons of parsley. Remove the chile from the tuna sauce, then stir the anchovy mixture into the sauce. Taste the sauce, and add salt if needed. Drain the spaghetti and toss with the sauce.