In a large pot melt the butter and stir in the onions. Also, add a dash of salt. . Let the onions cook on medium-low heat for 45 minutes, stirring occasionally.
Once the onions are brownish in color add the sherry and white wine to deglaze the pan. Scrape up any brown bits on the bottom of the pan.
Add the thyme, salt, and pepper. Pour in the pasta, beef broth, and Worcestershire sauce.
Bring to a boil and cook for 9 minutes, stirring occasionally.
Stir in the Parmesan cheese and half and half. Let the pasta sit 2-3 minutes to let the sauce thicken.