How to Make It
In a small bowl, whisk the mustard powder with the water. In a medium saucepan, combine the cherries with the vinegar, port and sugar and bring to a boil. Simmer over moderate heat until the cherries are very tender and the liquid is syrupy, about 8 minutes.
Transfer the cherries and their liquid to a blender and add the mustard paste. Puree until smooth. Season the sour cherry mustard with salt and let cool completely before serving.