In a large pot, bring salted water to a boil and cook the spaghetti 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. Cook, stirring occasionally, until onions are golden brown and caramelized, about 20-25 minutes.

Add balsamic vinegar and black pepper, stir to combine, and cook for an additional 2 minutes.

Add minced garlic to the skillet with the caramelized onions and cook for 1 minute until fragrant.

Pour in the heavy cream and bring to a simmer. Stir in the grated parmesan cheese until melted and smooth.

Add the cooked spaghetti to the skillet and toss to coat in the sauce. Stir in chopped parsley and adjust seasoning if necessary.

Serve immediately, garnished with additional parmesan cheese and parsley if desired.