1. In a large pot, melt the butter over medium heat. Add the sliced onions, sugar, and salt, stirring to coat the onions in butter.
2. Cook the onions, stirring occasionally, for about 30 minutes, until they are deeply caramelized and golden brown.
3. Add the minced garlic and cook for another minute until fragrant.
4. Pour in the white wine to deglaze the pot, scraping up any browned bits from the bottom. Simmer until the wine is mostly evaporated, about 5 minutes.
5. Add the beef broth, thyme, and bay leaf, then bring the mixture to a simmer. Cook for another 10 minutes to allow the flavors to meld.
6. While the broth is simmering, cook the pasta according to package instructions until al dente. Drain and set aside.
7. Remove the bay leaf from the broth, then stir in the cooked pasta. Season with salt and pepper to taste.
8. Ladle the pasta and broth into bowls, then top with grated Gruyère cheese. Broil briefly, if desired, to melt and lightly brown the cheese.
9. Garnish with chopped fresh parsley before serving.
