Preheat your oven to the lowest possible setting, typically around 150-175°F (65-80°C). Line a large baking sheet with parchment paper or a silicone mat. This step is crucial to prevent sticking.

In a medium saucepan, combine the chopped rhubarb, sugar, and water. Cook over medium heat, stirring occasionally, until the rhubarb breaks down and becomes very soft, about 10-15 minutes. Mash with a fork or potato masher to break up any larger pieces.

Allow the rhubarb mixture to cool slightly, then transfer it to a food processor or blender. Process until completely smooth. If you prefer a less tart fruit leather, you can add a little more sugar at this stage and blend again.

Spread the rhubarb puree evenly onto the prepared baking sheet. Aim for a thickness of about 1/8 to 1/4 inch. Ensure the edges are not too thin, as they will dry faster and become brittle.

Bake in the preheated oven for 4-6 hours, or until the fruit leather is no longer sticky to the touch and peels away easily from the parchment paper. The drying time will vary depending on your oven and the thickness of the puree.

Once dried, remove the fruit leather from the oven and let it cool completely. Once cool, peel it off the parchment paper, cut it into strips, and roll each strip in a piece of parchment paper for storage. Store in an airtight container at room temperature.