1 ounce yellow mustard seeds
1/4 dry white wine
1/4 cup white wine vinegar
1 tablespoon minced shallot
1/2‑teaspoon ground horseradish
1/2‑teaspoon kosher salt
1/4‑teaspoon white pepper
1. In a non-reactive bowl combine all the ingredients and refrigerate overnight.
2. The next day transfer the mixture to a mortar and pestle and work the mixture at he desired consistency. More vinegar may be necessary.
3. If you prefer a perfectly smooth texture the job goes quicker in a blender or mini food processor.