1. In a large skillet or Dutch oven, melt butter with olive oil over medium-low heat.

2. Add sliced onions, salt, and pepper. Cook slowly for 25–30 minutes, stirring often, until deeply caramelized and golden brown.

3. Stir in thyme and garlic, cooking for another 1–2 minutes until fragrant.

4. Deglaze the pan with white wine (if using), scraping up browned bits; cook until mostly evaporated.

5. Add broth and bring to a simmer.

6. Meanwhile, cook pasta in salted boiling water until al dente; reserve 1/2 cup pasta water, then drain.

7. Stir cream into the onion mixture, then add cooked pasta and toss well.

8. Add Gruyère and Parmesan cheeses gradually, stirring until melted and creamy. Add reserved pasta water if needed to loosen the sauce.

9. Season to taste with salt and pepper.

10. Serve hot, garnished with fresh parsley and extra cheese.