In a large skillet brown sliced sausage until caramelized. Remove from skillet and set aside, leaving the drippings from the sausage in the pan.
Lower heat to medium-low and place onions and salt in sausage drippings. Cook onion, stirring often, until very soft and well caramelized-- about 15 minutes.
Boil pasta according to packaged directions and drain, reserving ½ C of the starchy cooking water.
Add sausage to the onions as well as the tomatoes and oregano. Bring to a simmer and stir in pasta and parmesan cheese, adding a bit of the cooking water if necessary to loosen it up. Serve with additional parmesan cheese and chopped fresh basil if desired.