Caramelize the Onions:

1. Heat Butter and Oil: In a large oven-safe skillet or pot, melt the butter with the olive oil over medium heat.
2. Cook the Onions: Add the sliced onions, sugar, salt, and pepper. Cook for 20-25 minutes, stirring occasionally, until the onions are golden and caramelized. Lower the heat if they start to burn.
3. Add Garlic and Thyme: Stir in the minced garlic and thyme, cooking for another minute until fragrant.

Prepare the Pasta:

4. Add Broth and Cream: Pour in the beef or vegetable broth and heavy cream, stirring to combine.
5. Cook the Pasta: Add the uncooked pasta to the pot, ensuring it’s submerged in the liquid. Cover and simmer on medium-low heat for 10-12 minutes, stirring occasionally, until the pasta is just al dente.

Assemble and Bake:

6. Add the Cheese: Stir in half of the gruyere and Parmesan cheese until melted and creamy.
7. Top with Cheese: Sprinkle the remaining gruyere and Parmesan cheese evenly over the top.
8. Bake: Transfer the skillet to a preheated oven and broil on high for 3-5 minutes, or until the cheese is bubbly and golden.

Garnish and Serve:

9. Garnish: Remove from the oven and sprinkle with fresh parsley for a pop of color.
10. Serve Hot: Let the pasta bake cool slightly before serving straight from the skillet.