Rinse the tomatoes under cold water. Remove the core and some seeds, and then cut into large bite-sized pieces.
In a large skillet over medium heat, sauté the onion in olive oil until golden. Add the tomatoes and stir together for a few minutes. After a few minutes, add the basil leaves and salt and pepper to taste.
Remove from heat.
Meanwhile, bring a large pot of salted water to a boil. Cook the pasta until al dente.
Drain, and mix into the skillet with the tomatoes.