Creamy French Onion Pasta

Cook the rigatoni pasta in boiling salted water according to package directions until it is al dente.
While the pasta is cooking, heat a large skillet over medium heat with the butter.
When the butter melts, add the onions, salt, pepper, and vinegar and toss to combine.
Continue to saute the onions, stirring frequently, for 12-15 minutes until the onions are a deep golden brown color. They should be softened but still have a slight bite to them.
Drain the pasta from the cooking water when it's al dente and set aside.
Add the fresh thyme leaves to the onions and saute for 1 minute.
Deglaze the pan by adding the beef stock and scraping the bottom of the pan to remove any stuck-on bits.
Add the heavy cream and gruyere cheese to the sauce and remove off the heat.
Stir the sauce until the cheese is melted and then toss with the drained pasta.
Serve with extra gruyere cheese on top, if desired.