Instructions

1. Use a banana with freckles across the skin. Below that stage it is still largely starch, and starch dries chalky where sugar dries chewy.
2. Mash the banana into the raspberries with a fork instead of blending the pair. A blender whips banana into a pale foam and the red disappears into it.
3. Stir the jam and the lemon juice in last. The jam is already cooked and set so it only needs folding through, and the lemon is what keeps the banana from browning while you work.
4. Spread the puree at 1/8" across the middle of the sheet and build the rim up a little higher. The rim finishes first, and that extra depth is what stops it going crisp while the centre is still soft.
5. Get it onto the tray promptly once it is mixed. Banana starts darkening the moment it is mashed, and the sheet holds whatever color it had going in.
6. Dehydrate at 135 °F for 9 hours. The jam brings extra sugar with it, which keeps this leather pliable but also lets it feel wet at the thick spots long after the rest is done.
7. Check the raised rim and the deepest part of the centre rather than a flat patch in the middle. When both are leathery and take no print, it is finished.
8. Peel it warm, roll it up in parchment paper, and keep the rolls in a sealed jar somewhere cool for a month. Raspberry seeds make this sheet more fragile than most, so roll it loosely or it splits along the fold.