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.
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.
Stir in the Parmesan until the cheese is melted and everything is well blended. Transfer to a serving bowl and serve hot.
Notes
This recipe calls for half and half, which is a middle ground between milk and heavy cream. You can use milk (whole, please nothing lower fat), half and half, or cream in this recipe, depending on how rich you are looking for your dish to be. You can also use half milk and half cream, or any combo of the aforementioned dairy products. Whatever you have on hand should work; the level of richness will depend largely on how much fat is in your dairy.