In a large skillet, heat the olive oil and add the onion. Puree the tomatoes and when the onion is golden, add to the skillet. Reduce to simmer and let cook for about 10 minutes. Add salt and pepper to taste. Mix in the mascarpone.
Meanwhile, bring a large pot of salted water to a boil. Cook the pasta until al dente, drain it, and add to the skillet. Cook together for about a minute and serve immediately with a sprinkle of Parmigiano.