01 - Bring a large pot of salted water to a boil. Cook the pasta according to package directions until al dente. Reserve 1/2 cup pasta water, drain, and set aside.
02 - Melt butter in a large skillet over medium heat. Add the finely chopped onion and sauté for 3 to 4 minutes until soft and translucent.
03 - Stir in the minced garlic and cook for 1 minute until fragrant, stirring constantly to prevent burning.
04 - Add the chopped spinach and cook for 2 to 3 minutes until wilted and tender.
05 - Pour in the heavy cream and whole milk, stirring to combine. Bring mixture to a gentle simmer.
06 - Stir in grated Parmesan, salt, black pepper, and nutmeg if using. Simmer for 2 to 3 minutes until sauce thickens slightly.
07 - Add the drained pasta to the skillet and toss to coat evenly. Add reserved pasta water gradually, as needed, to achieve desired sauce consistency.
08 - Taste and adjust seasoning if necessary. Serve immediately, garnished with extra Parmesan cheese and black pepper if desired.