How to Make It
Fill a large pot with water, cover and bring to a boil. Add the canning jars, lids and rings along with a set of tongs and a ladle and simmer over low heat for about 10 minutes to sterilize. Cover the pot and turn off the heat.
Set a metal rack in another large pot. Fill the pot with water, cover and bring to a boil.
Meanwhile, in a saucepan, combine both vinegars with the brown sugar and salt and bring to a boil to dissolve. Using the sterilized tongs, remove the jars from the hot water and transfer them to a rimmed baking sheet.
Pack the plums, cloves, star anise, cinnamon, peppercorns and fennel seeds into the jars, leaving 1/2 inch at the top. Ladle the hot brine over the plums, stopping 1/2 inch from the top. Using the tongs, screw the lids and rings on the jars securely but not too tightly.
Using canning tongs, lower the jars onto the rack in the boiling water, making sure they are covered by 1 inch of water. Boil over high heat for 20 minutes. Using the canning tongs, transfer the jars to a rack to cool until the lids seal (they will be concave); refrigerate any jars that do not seal. Store the sealed jars in a cool, dark place for up to 6 months.