Because they use modernist ingredients, these amounts are given in metric by weight. For more information on how to measure modernist ingredients check out this article.
For the Watermelon Soup
Place all the ingredients in a blender or food processor. Processes until everything is broken down and the soup is nice and smooth. Store the soup in the refrigerator until ready to serve, it will keep for several days.
For the Pickled Rind
Trim the watermelon off the rind, leaving just a bit of the red flesh. Use a vegetable peeler to remove the tough, outer layer of green. Cut the watermelon rind into 1.5" x 0.5" (37mm x 12mm) strips.
Make a brine by combining the salt and 750 grams of water in a bowl, mixing until the salt is dissolved. Add the watermelon rind and let soak for at least 12 to 24 hours in the refrigerator. Drain the watermelon rinds and rinse off the excess brine.
Add the brown sugar, apple cider vinegar, and water to a pot. Heat over medium heat until the sugar dissolves. Add the watermelon rind and jalapeños then bring to a boil. Reduce to a simmer and cook for 5 minutes then remove from the heat and let cool.
You can store the pickled rinds in any air-tight container like mason jars or Tupperware. Let them rest for another 12 to 24 hours for the flavors to mellow before serving.
To Assemble the Watermelon Soup and Rind
Fill a bowl or small glass with the watermelon-jalapeño soup. Top with a few mint leaves and a pickled watermelon rind.