Heat a large and preferably deep skillet over medium heat. Add the spinach in handfuls, stirring after each. This step should not take more than a minute.

Once the spinach is wilted, add the butter and reduce the heat. Quickly season with salt, pepper, and nutmeg, and stir. Add minced garlic and mix again. This step should not take more than a minute, so that the butter and garlic are not burned.

Finally, stir in the half-and-half and heavy whipping cream, and bring to a boil. When the sauce begins to bubble slightly, add the Parmesan cheese and keep stirring until the ingredients are thoroughly combined and the flavors come together, about 2 minutes. Serve immediately.