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.
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 (about 2 minutes less than the package recommends). 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.
