Tip the strawberries and apple into a saucepan. Put the pan over a low heat, you don’t need to add any water. If you've used frozen fruit, juice will run out as they thaw. Heat gently, keeping an eye on the pan until the strawberries start to break down, you can put a lid on the pan if you like, this keeps the steam in and helps the fruit break down. Cook until the strawberries have softened and given up their juice and the apple is soft.
Take the pan off the heat. Purée the contents with a stick blender or in a food processor. Squeeze the purée through a sieve into a clean pan, pushing as much pulp through as possible.
Put the pan back on a low heat and cook, stirring occasionally, until the purée thickens enough to leave a clear patch on the base of the pan when you pull a spoon through it. The more moisture you drive off now, the shorter your drying time will be.
Heat the oven to as low as it will go. Pour the purée onto a lined baking sheet (we use a reusable silicone tray liner) and spread it out to a thin layer, the purée should be thick enough to look opaque.
Put the tray in the oven and dry until the surface is no longer tacky when you touch it, but the leather is still flexible – don’t overcook it or it will turn brittle. This will take several hours depending on the oven temperature. If it is very hot and sunny you can air-dry the leather, or use a dehydrator.
Leave the leather to cool to warm and then peel it carefully off the tray. Trim off the raggedy edges. Roll the leather up, then cut the roll into sections, using scissors is easiest. Each of these will unroll to make a strip. Keep the strips individually wrapped and airtight. The offcuts can be chopped up and used in baking, or snacked on.