Bring a large pot of salted water to a boil. Add 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 and salt, cooking slowly until onions are golden brown and caramelized, about 20-25 minutes. Stir occasionally.

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

Stir in heavy cream and grated parmesan cheese, mixing until the cheese is melted and the sauce is creamy. Season with black pepper and red pepper flakes if using.

Add the cooked pasta to the sauce, tossing to coat evenly. Serve hot, garnished with fresh parsley.