- Add the zucchini and onion to a large bowl. Add salt and toss to combine. Add enough ice water to cover, stirring until salt dissolves. Leave to stand, at room temperature for about 1 hour.
- Drain and dry thoroughly between two towels. Return the zucchini and onion to a dry bowl.
- In a saucepan, combine vinegar, sugar, dry mustard, mustard seeds, and turmeric, over medium heat and simmer for 3 minutes. Remove from the heat, and let stand until just warm.
- Pour the cooled liquid over the zucchini and onion mixture, stirring to combine. Transfer to sterilised jars. Cover, and refrigerate for 1 to 2 days before serving. They will keep for about 3 months, refrigerated, in an airtight container.