01 - Line an 8x8-inch baking pan with parchment paper, allowing overhang on all sides for easy removal later.
02 - Place 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.
04 - Pour the pureed mixture into the prepared pan. Lift and tap the pan gently on the counter to release air bubbles and create an even surface.
05 - Place the pan in the freezer for approximately 4 hours, or until the mixture is completely firm and set.
06 - Use the parchment paper overhang to lift the frozen slab from the pan. Cut into 12 equal bars using a sharp knife, running the knife under warm water between cuts if necessary.
07 - Top bars with fresh chopped mint and lime wedges immediately before serving, if desired. Serve frozen.