01 - Wash and trim the spinach leaves. Roughly chop if using fresh spinach.
02 - Melt unsalted butter in a large skillet over medium heat. Add finely chopped onion and sauté for 2 to 3 minutes until translucent.
03 - Add minced garlic and cook for 1 minute until fragrant.
04 - Add spinach to the skillet. Stir continuously until wilted: about 3 to 4 minutes for fresh, or 2 minutes if using thawed frozen spinach.
05 - Pour in heavy cream and stir to combine. Allow to simmer gently for 5 minutes until the sauce slightly thickens.
06 - Stir in grated Parmesan cheese and ground nutmeg. Season with salt and black pepper to taste. Simmer for an additional 1 to 2 minutes until the sauce becomes creamy.
07 - Serve warm as a side dish or atop rice, pasta, or toast.