Puree the tomatoes in a blender. In a large skillet over medium heat, sauté the onion in the olive oil until golden. Add the tomatoes, salt, pepper and basil. Lower heat and simmer for about 20-30 minutes, until reduced.
Meanwhile, bring a large pot of salted water to a boil. Cook the pasta until al dente. Drain it well and add it to the sauce, and mix well over medium heat.
Serve immediately with a spoonful of stracciatella.