Preheat the oven as low as it goes with a rack in the middle. Line a baking sheet with a piece of parchment paper or a silicone baking mat and spray lightly with cooking spray . In a blender, whiz all ingredients together to form a relatively smooth puree. Pour the puree onto the lined baking sheet and tilt the pan to form a thin layer that is as even as possible. Bake the puree until it is still slightly tacky to the touch but peels up fairly easily from the parchment. Depending on the fruit you use and its moisture content, this can take anywhere from 2 to 8 hours, so use your senses.
