In a large saucepan, melt butter over medium-low heat.
Add onion and garlic; saute' for about 10 minutes, or until onion is translucent.
Stir in salt,pepper and nutmeg.
Sprinkle flour over onion and cook over low heat, stirring with wooden spoon, about 4 minutes or until there is a nutty aroma.
Add half and half and cook over med-low heat, whisking occasionally, until hot.
Raise heat to med-high and bring to a boil.
Whisk continuously 6-8 minutes, or until mixture thickens.
Remove pan from heat. Using handheld immersion blender, blend sauce for 1 to 2 minutes or until smooth.
Stir in spinach until well-mixed and heated through.
Just before serving, set over med heat,add Parmesan and stir until well-mixed and heated through.