1. Add butter and flour to a skillet and heat until hot and thick, about 3 minutes. Add onion and garlic and cook for 1 minute more.
2. Add milk and heat on low, whisking frequently until thickened. Season with salt, pepper and nutmeg.
3. In a separate skillet, add butter and spinach and heat until wilted.
4. Add to the cream mixture and mix well. Cook on low until heated through. Serve immediately.