Put the spinach in a colander and run cool water over it to thaw. Squeeze out as much moisture as possible. Set aside.
Melt the butter in a medium saucepan. Add the garlic and cook for 1 minute. Add the flour and nutmeg and cook, stirring, for 2 minutes. Gradually add the milk and cook, stirring, until thickened.
Add the spinach to the sauce mixture and continue cooking until the mixture is hot and the spinach is tender. Add the Parmesan cheese and continue cooking for 1 to 2 minutes. Taste and season with salt and pepper.