Combine cranberries, water and salt in a small saucepan over medium heat. Cook for 10-15 minutes, or until the cranberries have softened and popped open. Stir occasionally.
Stir in the mustard and maple syrup, and cook for an additional 5 minutes. Remove from heat.
Using an immersion blender, blend the sauce until smooth. A regular blender will work as well. Be sure to allow for the hot steam to escape the blender or you'll have a big mess on your hands!
Chill in the fridge until ready to serve.