01 -
Process watermelon chunks in a blender or food processor until smooth, then strain through a fine mesh sieve to extract the juice. Set aside measured quantity.
02 -
Chop strawberries and juice the lemon, ensuring all components are ready for assembly.
03 -
In a large heavy-bottomed pot, combine watermelon juice, strawberries, lemon juice, pectin, and butter (if using) over medium-high heat.
04 -
Stir the mixture gently but frequently until it reaches a rapid boil that does not subside while stirring.
05 -
Add sugar and continue to stir frequently. Once the mixture returns to a full rolling boil, start a timer and boil for 70 seconds, stirring constantly.
06 -
Take the pot off the heat. For refrigerator storage, ladle jam into sterilized jars leaving 5 mm headspace, allow to cool, seal, and refrigerate.
07 -
If preserving, submerge sterilized jars in boiling water ensuring 2.5 cm of water above. Process for 10 minutes, remove, and cool on towels until seals 'pop'. Store sealed jars at room temperature for up to 18 months.