01 - Heat the butter in a large skillet over medium heat. Add the chopped onion and cook for 3 to 4 minutes until translucent.
02 - Stir in the minced garlic and cook for 1 minute until fragrant.
03 - Add the spinach in batches, stirring continuously until wilted and excess liquid evaporates, about 4 to 5 minutes.
04 - Pour in the heavy cream and lower the heat to simmer. Cook for 3 minutes until the mixture thickens slightly.
05 - Mix in the grated Parmesan cheese and ground nutmeg. Season with salt and freshly ground black pepper to taste.
06 - Continue cooking for 2 to 3 minutes until the sauce is creamy and coats the spinach evenly.
07 - Serve immediately as an elegant and flavorful side.