Bring a large pot of salted water to a boil. Add the spaghetti and cook according to package instructions until al dente. Drain and set aside.

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

Add minced garlic to the caramelized onions and cook for an additional 1-2 minutes until fragrant. Pour in the vegetable broth and bring to a simmer.

Stir in the heavy cream and Parmesan cheese, mixing well until the cheese is melted and the sauce is creamy.

Add the cooked spaghetti to the skillet and toss to coat the pasta in the sauce. Cook for an additional 2-3 minutes to heat through.

Serve hot, garnished with fresh parsley.