Caramelize the Onions:

1. In a large pot or deep skillet, heat butter and olive oil over medium heat.
2. Add sliced onions, ½ teaspoon salt, black pepper, and sugar. Cook, stirring occasionally, for 20-25 minutes until deeply golden brown.
3. Stir in garlic and thyme, cooking for 1 more minute.

2. Deglaze & Simmer:

1. Pour in the white wine (if using) and scrape the bottom of the pan to release any browned bits. Cook for 2 minutes until reduced.
2. Add beef broth, Worcestershire sauce, and pasta. Bring to a simmer, stirring occasionally to prevent sticking.

3. Cook the Pasta:

1. Let the pasta cook in the broth for 10-12 minutes (or until al dente), stirring occasionally.
2. Once most of the liquid is absorbed, stir in heavy cream, Gruyère, Parmesan, and Dijon mustard. Mix until creamy and smooth.

4. Serve & Enjoy:

1. Garnish with fresh parsley and extra Parmesan.
2. Serve immediately with crusty bread or garlic toast.