In a skillet, heat the oil. Add the fennel, cover and cook over low heat, stirring occasionally, until softened, 25 minutes. Add the vinegar and cook, uncovered, until the fennel is tender, 15 minutes. Transfer to a blender. Add the water and mustard powder and puree. Scrape into a bowl and let cool. Stir in the whole-grain mustard and season with salt. Transfer to jars, cover and refrigerate.