In a blender add watermelon and puree until smooth. Add the frozen strawberries, 1 Tbsp. of the sugar, lime juice, mint leaves, and pinch of salt. Blend until smooth. Taste. Note: If you prefer a sweeter drink, add the remaining sugar and blend again to combine.
For individual serving, pour 6 ounces of the strawberry watermelon mixture to a tall glass and top with 1 to 2 ounces of club soda. Stir to combine. Top with a mint leaf.