- 2 cups/480 ml apple cider vinegar
- 1 cup/240 ml water
- 1 onion, diced
- 1/2 cup plus 2 tbsp /75 g firmly packed dark brown sugar
- 2 tsp fine sea salt, such as fleur de sel or sel gris
- One 3-in/7.5-cm cinnamon stick
- 6 whole allspice berries
- 6 whole cloves
- 2 tsp ground turmeric
- 1/2 cup/100 g yellow mustard seeds
- 1/4 cup/50 g brown mustard seeds
- Combine the vinegar, water, onion, brown sugar, salt, cinnamon stick, allspice, cloves, and turmeric in a medium saucepan. Bring the mixture to a boil over medium-high heat. Allow the liquid to boil rapidly to reduce the volume by half, about 10 minutes.
- Put the yellow and brown mustard seeds in a medium heat-proof bowl. Strain the hot vinegar mixture through a fine-mesh strainer into the bowl of mustard seeds. Use a heat-proof silicone spatula to press the onions into the strainer to extract all of the liquid. Push the mustard seeds down to completely submerge them in the liquid, cover, and refrigerate for 24 hours.
- Transfer the seeds and liquid to a food processor and process until it forms a smooth paste. Put the mustard in an airtight container and refrigerate for at least a 3 days, and preferably 1 week, before serving to allow its pungency to mellow. After about 2 weeks, the mustard is at its prime. It will keep, refrigerated, for several months. If the mustard becomes too thick as it matures, thin it out by adding a splash of vinegar or water.