Preheat your oven to 350°F (175°C). Cook the pasta according to package instructions until al dente. Drain and set aside.
In a large skillet, heat the olive oil over medium heat. Add the sliced onions and sugar, cooking for about 15-20 minutes until the onions are caramelized and golden brown, stirring occasionally.
Add the minced garlic to the skillet and cook for an additional minute until fragrant.
Pour in the beef broth, Worcestershire sauce, and thyme. Season with salt and pepper. Bring to a simmer and let it cook for 5 minutes.
Stir in the heavy cream and cooked pasta, mixing well to combine all ingredients.
Transfer the pasta mixture to a greased 9x13-inch baking dish. Top with Gruyère and mozzarella cheese.
Bake in the preheated oven for 20-25 minutes, or until the cheese is bubbly and golden brown.
Remove from the oven and let it cool for a few minutes. Garnish with fresh parsley before serving.