Step 1: Caramelize the Onions

1. Heat the butter and olive oil in a large, heavy-bottomed pot or Dutch oven over medium heat.
2. Add the sliced onions and stir to coat them in the fat. Cook for 10 minutes, stirring occasionally.
3. Sprinkle the onions with sugar (if using) and season with salt and black pepper. Continue cooking for another 15-20 minutes, stirring frequently, until the onions are golden brown and caramelized. Reduce the heat if necessary to avoid burning.

Step 2: Add Garlic and Thyme

1. Stir in the minced garlic and dried thyme. Cook for 1-2 minutes, until fragrant.

Step 3: Build the Broth

1. Pour in the beef broth and water, scraping the bottom of the pot to deglaze and release any browned bits.
2. Bring the mixture to a gentle boil.

Step 4: Cook the Pasta

1. Add the pasta to the pot, ensuring it is submerged in the liquid. Reduce the heat to medium and simmer, stirring occasionally, for 10-12 minutes, or until the pasta is cooked al dente. If the liquid reduces too quickly, add an extra splash of water or broth.

Step 5: Add Cream and Cheese

1. Lower the heat to medium-low and stir in the heavy cream, Gruyère, and Parmesan cheese. Mix until the cheese is fully melted and the sauce is creamy.
2. Taste and adjust seasoning with additional salt and pepper, if needed.

Step 6: Serve and Garnish

1. Divide the pasta among serving bowls.
2. Garnish with fresh thyme or parsley and a sprinkle of extra Parmesan cheese. Serve with crusty bread or garlic bread for dipping.