Thomas Keller's lovely recipe:
In a skillet, heat 1 tablespoon vegetable oil. Add 3 cups chopped fennel, cover, and cook over low heat, stirring occasionally, until softened, 25 minutes. Add 1/2 cup white wine vinegar and cook, uncovered, until the fennel is tender. 15 minutes. Transfer the contents of the skillet to a blender. Add 1 tablespoon water and 1 teaspoon dry mustard powder and puree. Scrape the puree into a bowl and let cool. Stir in 2 teaspoons whole-grain mustard and season with salt. Transfer to jars, cover, and refrigerate.
This would be so delicious with grilled or roasted vegetables... or in a nice sandwich