Set a large pot over medium-low heat, then add the butter and olive oil. Once the butter melts, toss in the sliced onions, stirring frequently. Cook for about 35 minutes until they are caramelized with a deep golden color and jammy texture.

Add garlic and thyme. Stir in the minced garlic and fresh thyme, letting them cook for an additional 1 minute.

Deglaze with wine and sherry. Pour in the white wine and sherry, scraping the bottom of the pot to loosen any brown bits. Let it simmer for 2 minutes until it reduces by at least half.

Build the sauce. Add the Worcestershire sauce and black pepper, stirring to combine.

Cook the pasta. Pour in the beef-style broth, water, and uncooked pasta. Stir everything together, ensuring the pasta is submerged. Bring it to a boil, then reduce to low heat and simmer for about 15 minutes, until the pasta is al dente.

Finish with cream and cheese. Remove the pot from heat and gently stir in the heavy cream and shredded Gruyère cheese, letting it melt into the sauce.

Serve hot. Adjust seasoning to taste and serve hot.