01 - Bring a large pot of salted water to a boil. Add pasta and cook 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 sauté until soft and translucent, about 3 to 4 minutes.
03 - Incorporate minced garlic and cook for 1 minute until fragrant, stirring constantly to prevent burning.
04 - Add chopped spinach and cook for 2 to 3 minutes until wilted.
05 - Pour in heavy cream and milk, stirring to combine. Bring the 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 evenly. Add reserved pasta water gradually if a thinner sauce is desired.
08 - Taste and adjust seasoning if necessary. Serve immediately, optionally garnished with extra Parmesan and black pepper.