Add the frozen watermelon, banana, agave nectar, and lime zest to a food processor.
Pulse a few times to break the fruit into small pieces, then blend until completely smooth, pausing to scrape down the sides as needed. (No liquid required—the watermelon has plenty of water on its own!)
Taste the mixture and, if you’d like it sweeter, add a little more agave and blend again briefly.
Enjoy right away for a soft, scoopable texture, or transfer to a freezer-safe container and freeze for 1 hour (or longer) for a firmer, more gelato-like consistency. When ready to serve, scoop into bowls and top with more lime zest if desired.
Notes
Watermelon Note: Cube the watermelon and spread it out on a baking sheet lined with parchment paper to freeze. This will need to be done a day in advance (or longer) before making the gelato. Freezing the watermelon on the parchment rather than a container or bag keeps the watermelon chunks separate, making it easier to break up in the processor. After it’s frozen, then you can transfer the cubed watermelon to a resealable plastic bag or airtight container until ready to make the recipe.
Banana Note: Don’t like bananas? Their flavor isn’t pronounced here, and they do help make the gelato a little creamier. If you really want to omit it, you can; the finished texture will be a little grittier.
Idea: Want some flavor variations? Replace 1–2 cups of the watermelon cubes with 1–2 cups of frozen raspberries or strawberries!
Storage: Best enjoyed immediately, but can also be kept in the freezer and enjoyed later, just give it a chance to soften once thoroughly frozen before eating.