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1
Up to 24 hours before: add mustard seeds to vinegar, cover, and let sit at room temperature (you can get away with soaking the seeds for less time with perfectly fine results. If you're organized though, the longer soak time allows the seeds to plump up nicely.)
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2
Wash oranges thoroughly and cut zest off in strips, leaving off as much pith as possible. Dice zest strips. You should have approximately 1/3-1/2 cup of diced zest.
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3
Place the zest in a small pot with enough water to cover. Bring to a boil, then remove from heat and drain water. Cover with fresh water and repeat two more times. This will help to remove the bitterness from any pith remaining on the zest. Add 1/2 cup water and bring back to a boil (do not drain this time.)
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4
While zest is boiling, supreme the oranges (cut the flesh from between the membranes, leaving behind any pith and membrane.) Do this over a bowl to catch any juices. Squeeze the remaining membrane into the bowl to extract as much juice as possible.
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5
Coarsely chop the orange segments and add to the juice in the bowl. You should have about 1 cup between the fruit and the juice.
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6
To the zest in the pot add the orange segments and juice, soaked mustard seeds and vinegar, dried cranberries, sugar, mustard powder, chili flakes, and sea salt. Bring back to a boil.
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7
Boil gently until reduced and thick, about 20-30 minutes. Taste and adjust seasoning if necessary.
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8
Allow to cool and store in a jar in the refrigerator.
