In a large saucepan over medium heat, melt butter. Add the flour & whisk for 1 minute. Add 1/2 of the milk & whisk until smooth. Whisk in remaining milk. Bring to a boil. Add salt, pepper & nutmeg. Reduce heat & simmer for 10 minutes, stirring occasionally. Whisk in Dijon mustard until combined. Remove from heat & set aside.