In a large skillet, over medium heat, sauté the onion in the olive oil until golden. Add the tomatoes and cook for about 10 minutes, or until some of the tomato juices evaporate. Mix in the tuna, half of the parsley and salt and pepper to taste. Cook for five more minutes.
Bring a large pot of salted water to boil.
Cook the pasta until al dente (about 2 minutes less than package directions). Drain and add it to the tuna. Cook together for one minute.