Step 1: Prepare the plums by washing them, removing the pits, and roughly chopping them into 1-inch pieces.
Step 2: In a medium saucepan, combine the chopped plums, granulated sugar, and 1/2 cup of water. Bring the mixture to a gentle simmer over medium heat, stirring occasionally until the sugar dissolves.
Step 3: Reduce the heat to low and continue to simmer for 10-15 minutes, or until the plums are very soft and have broken down into a compote-like consistency.
Step 4: Remove the saucepan from the heat and let the plum mixture cool slightly for about 5-10 minutes.
Step 5: Carefully strain the plum mixture through a fine-mesh sieve into a clean bowl. Use the back of a spoon to press firmly on the solids, extracting as much liquid and flavor as possible. Discard the remaining plum pulp.
Step 6: Allow the strained plum syrup to cool completely to room temperature. This step is important before adding the vinegar.
Step 7: Once completely cool, stir in the 1/2 cup of apple cider vinegar until thoroughly combined. Taste and adjust for desired sweetness or tang.
Step 8: Transfer the plum shrub concentrate to a clean, airtight bottle or jar. Refrigerate for at least 24 hours to allow the flavors to meld. The shrub can be stored in the refrigerator for up to 2-3 weeks.
Step 9: To assemble the mocktail, fill a tall glass with ice cubes.
Step 10: Pour 1-2 tablespoons (or more, to taste) of the prepared plum shrub concentrate over the ice.
Step 11: Top the shrub with sparkling apple cider or juice, filling the glass to your desired level.
Step 12: Gently stir to combine. Garnish with a fresh plum slice or a mint sprig if desired, and serve your Plum Shrub & Sparkling Apple Mocktail immediately.