-Using a vegetable peeler, remove tough green rind from 1 whole watermelon; discard.
-Slice watermelon 1" thick.
-Cut away all but ¼" flesh from each slice; reserve flesh for another use.
-Cut rind into 1" pieces. (You should have about 4 cups.)
-Combine the following in a large saucepan over medium heat:
1 serrano chile, thinly sliced, seeds removed if desired
1, 1" piece peeled ginger, thinly sliced
2 star anise pods
4 tsp kosher salt
1 tsp black peppercorns
1 cup sugar
1 cup unseasoned rice vinegar
½ cup water
-Stir to dissolve sugar and salt.
-Add watermelon rind and return to a boil.
-Reduce heat and simmer until just tender, about 5 minutes.
-Remove from heat and let cool, setting a small lid or plate directly on top of rind to keep submerged in brine, if needed.
-Transfer rind and liquid to an airtight container; cover and chill at least 12 hours.