Cook the pasta: Bring a large pot of salted water to a boil. Add the pasta and cook according to the package instructions until al dente. Drain and set aside.

1. Caramelize the onions: While the pasta cooks, melt the butter in a large skillet or sauté pan over medium heat. Add the sliced onions and cook for 15-20 minutes, stirring occasionally, until the onions become golden brown and caramelized.
2. Sauté the garlic: Once the onions are caramelized, add the minced garlic to the skillet and cook for another 1-2 minutes until fragrant.
3. Add the broth and cream: Stir in the beef broth and heavy cream, scraping up any bits from the bottom of the pan. Bring the mixture to a simmer, then cook for 5-7 minutes, allowing the sauce to thicken slightly.
4. Add the cheese: Stir in the Gruyère cheese and Parmesan cheese, allowing them to melt and make the sauce creamy. Taste and adjust the seasoning with salt, pepper, and thyme.
5. Combine the pasta: Add the cooked pasta to the skillet with the onion sauce, tossing to coat the pasta evenly in the creamy, cheesy sauce. If the sauce seems too thick, add a bit more broth or cream to loosen it up.
6. Serve: Garnish with fresh parsley and an extra sprinkle of Parmesan cheese. Serve immediately.