Bring a large pot of water to a boil.

Add the tomatoes and blanch until the skins are loosened (about 30 seconds).

Transfer the tomatoes to a large rimmed baking sheet to cool.

Peel the cooled tomatoes and halve them crosswise.

Set a coarse sieve over a large bowl. Squeeze the tomato halves to release the seeds and juices.

Press on the seeds to extract as much juice as possible (approximately 2 cups).

Coarsely chop enough tomatoes to yield 4 cups.

Cut the remaining tomatoes into 1/2-inch dice.

In a food processor, puree the coarsely chopped tomatoes with the tomato juice, chopped watermelon (2 cups), and chopped cucumber (2 cups).

Transfer the soup to a large bowl.

Stir in the diced tomato, watermelon, and cucumber, along with sherry vinegar, 1 tablespoon of olive oil, salt, and pepper.

Refrigerate the soup until thoroughly chilled, at least 1 hour.

In a small bowl, combine thinly sliced scallions, minced jalapenos, chopped cilantro, and lime juice. Season with salt and pepper to create the relish.

Ladle the chilled soup into bowls.

Drizzle with the remaining 2 tablespoons of olive oil and sprinkle minced chives on top.

Serve the scallion relish alongside the soup.