01 - Melt the butter in a large skillet over medium heat. Add the chopped onions and sauté until soft and translucent, about 2 minutes.
02 - Stir in the minced garlic and cook for an additional 30 seconds until fragrant.
03 - Add the spinach in batches, stirring gently until all leaves have wilted and reduced in volume.
04 - Pour in the heavy cream and stir to blend evenly with the spinach and onion mixture.
05 - Reduce heat and simmer for 3 to 5 minutes to allow the cream to thicken slightly.
06 - Stir in the grated Parmesan cheese and nutmeg. Season with salt and black pepper to taste, cooking for another 1 to 2 minutes until the sauce coats the spinach luxuriously.
07 - Remove from heat and serve immediately while hot.