In a small saucepan, combine the 1/2 cup of water and the granulated sugar. Heat over medium heat, stirring constantly until the sugar is completely dissolved. Remove from heat and let the simple syrup cool slightly.

In a mixing pitcher, combine the cooled simple syrup, the guava nectar, and the fresh lemon juice.

Stir the guava-lemon concentrate thoroughly until well combined. Taste and adjust sweetness or tartness if necessary by adding a splash more lemon juice or simple syrup.

To serve, fill four tall glasses with ice cubes. Add approximately 1/4 of the guava-lemon concentrate mixture into each glass.

Top each glass with chilled sparkling water or club soda, leaving about half an inch of space at the top.

Gently stir the infusion in the glass. Garnish each serving with a thin slice of fresh lemon and a slice of guava.