Boil pasta as per directions in a large pot of salted water. Drain and set aside
Add oil in a large saucepan or skillet. Saute onions for 3-4 minutes. Add in garlic and salt. Continue cooking for 2 minutes.
Add in red pepper flakes and tomato paste. Cook down for 5-6 minutes until sauce darkens in color. Pour in vodka and let cook another 2-3 minutes. Add in cream and remove from heat. Stir together, adding in pasta water. Toss in parmesan and continue stirring. Add pasta and once coated with sauce, add butter, mixing until dissolved. Plate and garnish with basil or parsley if desired.