Got this off another forum and it sounds and looks good.
I used plain vinegar.
Homemade mustard recipes by Ashley English
Pickled Mustard Seed
Makes 2 1/2 cups
• 1 cup yellow mustard seed
• 1 1/2 cups cider or white wine vinegar + 1/3–1/2 cup, divided
• 1 teaspoon kosher salt
• 2 Tablespoons honey
• 2 teaspoons turmeric
1. Rinse mustard seeds and quickly drain. Pour into non-reactive saucepan and add vinegar and salt. Allow to soak overnight (or longer) uncovered at room temperature.
2. The next day, stir in the honey and turmeric (Foodie’s note: You can also add a huge variety of additional seasonings at this point: tarragon, black or green peppercorns, chipotles in adobo, ginger and lemongrass, etc.).
3. On the stovetop, bring the mixture to a simmer. Cook, stirring constantly with a wooden spoon, for 15–20 minutes. The seeds will expand and the mixture will bubble and sputter like polenta or grits, so stirring is important.
4. Remove from heat and cool to room temperature uncovered. Mixture will become thicker and denser.
5. Finally, stir in 1/3 to 1/2 cup more vinegar to thin the mixture and to freshen the flavor.
6. Store pickled mustard seeds in jars, refrigerated, up to a year.
Note: Mustard seeds typically continue absorbing liquid for weeks. If the mixture becomes too dense, simply stir in a few spoonfuls of vinegar.