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