Transfer to a blender and blend on high until smooth. Return back to the saucepan and cook for an additional 5-10 minutes on medium heat, stirring more frequently to avoid burning.
Notes
You'll know the mustard is done when it stays separated in the pan after scraping with a spoon.
Transfer to a storage container and chill in the refrigerator for at least 30 minutes.
Mustard will thicken some once chilled.
Keeps for up to a week in the fridge or up to three months in the freezer. I've also frozen, defrosted, and frozen again with no issues!