In a large skillet, heat the butter and olive oil over medium heat. Add the sliced onions, sugar, and salt. Cook, stirring occasionally, for about 20-25 minutes until the onions are golden brown and caramelized.

Once caramelized, set aside the onions.

In a large pot, bring water to a boil and cook the pasta according to package instructions until al dente. Drain and set aside.

In the same pot, add the beef broth and white wine (if using). Bring to a simmer.

Add the caramelized onions and cooked pasta to the broth. Stir to combine and heat through for about 5 minutes.

Ladle the pasta and broth into bowls. Top with a generous amount of gruyere and parmesan cheese.

Place under a broiler for 2-3 minutes until the cheese is bubbly and golden. Garnish with fresh parsley before serving.