Heat the olive oil in a large skillet. Sauté the onion until soft for about 3 - 5 minutes. Add the guanciale and garlic. Sauté for another 2 - 3 minutes. Add the tomato passata. Season with salt and pepper. Stir and let simmer over low heat for about 30 minutes.Cook the pasta. Drain, saving 1 cup of cooking water.Remove the sauce from the heat and stir in the heavy cream and parmigiana reggiano. Add the pasta to the skillet, adding cooking water as needed. Top with parsley and additional cheese.