Step 1: Heat olive oil and butter in a large Dutch oven or heavy-bottomed pot over medium-low heat. Add the thinly sliced onions, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Cook, stirring occasionally, for 20-30 minutes, or until the onions are deeply golden brown, very soft, and caramelized. Be patient, this step is crucial for flavor.
Step 2: Add the minced garlic and fresh thyme to the pot. Cook for another minute until fragrant, stirring constantly to prevent burning.
Step 3: If using, pour in the dry white wine. Scrape up any browned bits from the bottom of the pot with a wooden spoon. Let it simmer for 2-3 minutes until the liquid has mostly evaporated.
Step 4: Add the beef broth and dried pasta to the pot. Stir to ensure the pasta is submerged. Bring the mixture to a boil, then reduce the heat to medium-low. Cover the pot and simmer for 10-12 minutes, or until the pasta is al dente and most of the liquid has been absorbed. Stir occasionally to prevent sticking.
Step 5: Remove the pot from the heat. Stir in the heavy cream and half (1/2 cup) of the grated Gruyère cheese. Stir continuously until the cheese is completely melted and the sauce is smooth and creamy.
Step 6: Taste and adjust seasonings as needed with additional salt and pepper. Serve immediately, garnished with the remaining Gruyère cheese and fresh parsley or chives, if desired.