Puree the tomatoes in a blender or pass them through a food mill. In a large skillet over medium heat, sauté the onion in the olive oil until golden. Add the tomatoes, and salt and pepper and the basil. Reduce 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 then mix well over medium heat. Serve immediately with a generous sprinkle of ricotta salata.