Peel and discard the husks of the fruit. Rinse clean under cool water.
Roughly chop the fruit and blend in a blender or food processor until smooth.
Pass through a strainer and discard the seeds.
For the drink:
Muddle the mint, lime juice, tomatillo juice, and sugar in the bottom of a cocktail shaker, cup, or pitcher.
Add the ice and rum. If using a cocktail shaker, place the lid on it and shake. Pour everything into a highball glass – you want to keep the mint and ice in the drink! If using a cup or pitcher, just stir together.
Top with club soda. Garnish with an extra sprig of mint if desired, and serve!
The proportions in this recipe are for one cocktail. To make a pitcher, multiply the ingredients by four (or more!)