Cut fresh watermelon into chunks and blend on high until smooth. Strain the juice using a fine mesh strainer or cheesecloth.

In a small saucepan, combine watermelon juice and sugar, heating over low-medium until sugar fully dissolves. Cool in the refrigerator for at least 20 minutes.

Layer thin lemon slices along the sides of a large glass jar, then add pebble ice to the jar.

Pour the cooled watermelon syrup into the jar over the ice and lemon slices. Add freshly squeezed lemon juice and gently stir.

Top off the mixture with sparkling water, stirring gently. Serve immediately in chilled glasses filled with pebble ice.

Garnish each glass with additional watermelon slices and a lemon wedge on the rim.