Heat the olive oil and butter in a large skillet over medium-low heat. Once the butter has melted, add the sliced onions and 1 teaspoon salt. Cook for 25–30 minutes, stirring occasionally, until the onions are deeply golden, soft, and caramelized.
If the onions start sticking to the pan or browning too quickly, add a small splash of water and scrape up any browned bits from the bottom.
Meanwhile, cook the spaghetti according to the package directions until al dente. Before draining, reserve ¾ cup of pasta water.
Add the minced garlic to the caramelized onions and sauté for 30–60 seconds, until fragrant.
Add the reserved pasta water, heavy cream, onion powder, paprika, and Worcestershire sauce. Stir well and let the sauce gently simmer for 2–3 minutes.
Reduce the heat to low and gradually stir in the freshly grated Parmesan until melted and creamy. Season with additional salt to taste.
Add the cooked spaghetti directly to the skillet. Toss until the noodles are evenly coated in the creamy caramelized onion sauce. If the sauce is too thick, add another splash of pasta water.
Top with freshly chopped parsley or dried parsley, if desired. Serve immediately and enjoy!