Bring a large pot of salted water to a boil. Add the spaghetti and cook according to package instructions until al dente, usually about 8-10 minutes. Stir occasionally to prevent sticking.

Once cooked, reserve 1 cup of pasta water, then drain the spaghetti in a colander and set aside.

In a large skillet, heat the olive oil and butter over medium heat. Add the sliced onions and sprinkle with salt and sugar. The salt helps to draw out moisture from the onions, while the sugar aids in caramelization.

Cook the onions for about 20-25 minutes, stirring occasionally. You want them to soften and turn a deep golden brown. If they start to stick, lower the heat slightly and add a splash of water to deglaze the pan.

Once the onions are caramelized, add the minced garlic to the skillet and cook for an additional 1-2 minutes until fragrant. Be careful not to burn the garlic as it can become bitter.

Pour in the heavy cream and stir to combine. Allow the mixture to simmer for about 5 minutes until it thickens slightly.

Stir in the grated parmesan cheese and black pepper, mixing until the cheese is melted and the sauce is creamy. If the sauce is too thick, add reserved pasta water a little at a time until desired consistency is reached.

Add the drained spaghetti to the skillet with the caramelized onion sauce. Toss to coat the pasta evenly with the sauce.

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