- Melt butter in a large saucepan over medium heat.
- Add flour and cook, whisking occasionally, about 4 minutes.
- Quickly whisk in milk and season with salt, pepper, and nutmeg.
- Reduce heat to medium-low and cook, whisking occasionally, until smooth and velvety, about 25 minutes.
- Remove from heat and -if using- whisk in Parmesan.
- Transfer to a bowl, cover with plastic wrap, and use within 1 hour or chill.