Purée cucumbers, watermelon, bell pepper, and green onion (if using) in the food processor until smooth. Remove to a medium bowl, using a spatula to get the food processor as clean as possible.

Finely chop the parsley, mint, and cilantro (if using) in the food processor.

Stir the chopped herbs and spices into the soup. If desired, add a small amount of sea salt and lime juice (if using). Refrigerate until very cold. You can add some diced watermelon to give the soup more texture.

Roll up the basil leaves and thinly slice (chiffonade). Stir into the oil in a small saucepan.

Heat over low heat for 10 minutes. Turn off heat and let sit at least 20 minutes or until cool.

Strain into a clean glass jar.

Use a spoon or eye dropper to drizzle oil over soup when served.