Step 1

In a large deep skillet, melt the butter over medium-high heat. Add the onion and sauté for about 3 minutes until the onion is golden and starting to become tender. Add the spinach by big handfuls, cooking as you go, to wilt down the spinach. When all of the spinach has been added, stir for another minute to allow any liquid released by the cooking spinach to evaporate. Pour in the half and half, and bring to a simmer, stirring frequently. Season with salt and pepper, nutmeg (if using), and the cayenne pepper or red pepper flakes, and continue stirring until everything is well combined and creamy, about 3 more minutes.

Step 2

If you like (this is optional, but adds more creaminess) you can run an immersion blender through the spinach mixture a few times to puree some of the spinach, or scoop about ¼ of the spinach mixture into a food processor or blender and pulse a few times to make it a bit smoother. Return the blended spinach to the pot, if needed.

Step 3

Stir in the Parmesan until the cheese is melted and everything is well blended. Transfer to a serving bowl and serve hot.