01 - Line an 8x8-inch baking pan with parchment paper, allowing overhang on all sides for easy removal of the frozen bars.
02 - Combine watermelon cubes, lime juice, lime zest, and sugar in a blender. Process until completely smooth and no chunks remain.
03 - Taste the mixture and add more sugar or lime juice as needed to achieve desired sweetness and tartness balance.
04 - Pour the smooth puree into the prepared pan. Lift and gently tap the pan on the counter to release air bubbles and create an even surface.
05 - Place the pan in the freezer for 4 hours or until completely firm and set throughout.
06 - Using the parchment overhang, lift the frozen block from the pan. Cut into 12 equal rectangular bars with a sharp knife, rinsing the blade under warm water between cuts if necessary.
07 - Arrange bars on a serving platter. Top with fresh chopped mint and lime wedges just before serving, if desired. Serve immediately while frozen.