How to Make It
In a medium saucepan, combine everything except the mustard and salt and bring to a boil. Simmer over moderate heat until the peppers and onion are soft, about 5 minutes. Transfer to a blender and puree until very smooth. Add the Dijon and puree until just incorporated. Season the mustard with salt. Transfer to a bowl and let cool completely, then refrigerate until well chilled, about 30 minutes.