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