In a large skillet, melt the butter and olive oil over medium heat. Add the sliced onions and salt, cooking until the onions are caramelized, about 15-20 minutes, stirring occasionally.

Once caramelized, add thyme and black pepper, stirring to combine.

In a large pot, bring beef broth to a boil. Add pasta and cook according to package instructions until al dente.

Drain the pasta, reserving 1 cup of the broth.

Add the cooked pasta to the skillet with the caramelized onions. Pour in the reserved broth and stir to combine.

Top with gruyère and parmesan cheese, then cover and cook on low heat until the cheese melts, about 5 minutes.

Serve hot, garnished with fresh parsley.