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1
Stir watermelon rind, onion, and salt together in a large bowl; refrigerate until liquid has been released from the rind, about 24 hours.
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2
Combine vinegar, sugar, ginger, garlic, cinnamon, mustard, turmeric, chipotle pepper, and cloves in a large saucepan; bring to a boil. Decrease heat to low and simmer until sugar is dissolved, about 10 minutes. Add watermelon rind mixture and simmer for 30 minutes more.
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3
Sterilize the jars and lids in boiling water for at least 5 minutes. Pack piccalilli into hot, sterilized jars, filling to within 1/4 inch of the top. Run a knife or thin spatula around the insides of the jars after they have been filled to remove any air bubbles. Wipe the rims of the jars with a moist paper towel to remove any food residue. Top with lids and screw on rings.
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4
Place a rack in the bottom of a large stockpot and fill halfway with water. Bring to a boil and lower jars into the boiling water using a holder. Leave a 2-inch space between the jars. Pour in more boiling water if necessary to bring the water level to at least 1 inch above the tops of the jars. Bring the water to a rolling boil, cover the pot, and process for 15 minutes.
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5
Remove the jars from the stockpot and place onto a cloth-covered or wood surface, several inches apart, until cool. Once cool, press the top of each lid with a finger, ensuring that the seal is tight (lid does not move up or down at all). Store in a cool, dark area.
