Make the mint simple syrup: In a saucepan, combine 1 cup water and 1 cup sugar. Heat over medium, stirring until sugar dissolves. Bring to a simmer, then remove from heat. Add mint leaves, steep for 15-20 mins. Strain and cool completely.

Prepare guavas: Wash and halve guavas. Scoop out and discard the seeds. Reserve the guava skins.

Make guava puree: Place guava flesh in a blender. Blend until smooth. Strain through a fine-mesh sieve into a large pitcher to remove any remaining seeds.

(Optional) For deeper flavor, steep the reserved guava skins in the warm mint syrup for 10-15 minutes. Strain again to remove skins.

Juice lemons until you have 1 cup of fresh lemon juice.

Combine guava puree and lemon juice in the pitcher. Stir to combine.

Sweeten to taste: Gradually add the cooled mint simple syrup to the guava-lemon mixture, stirring and tasting as you go, until desired sweetness is reached.

Add 4 cups of cold water to the pitcher and stir well. Refrigerate for at least 1 hour to chill and let flavors meld.

Serve over ice, garnished with lemon slices, guava rounds, and a sprig of mint.