Preheat the oven to 50C
Put the apples in a saucepan with a lid and cook over a medium heat for about 5 minutes. Add strawberries and cook a further 10 minutes, until all completely soft. Mash with a fork and when cooled, push the pulp through a sieve.
Line a baking tray with greaseproof paper and give it light spray with oil. Pour the pulp on to the baking sheet and spread out to the corners.
Place in warm oven overnight (or about 12 hours). It should feel leathery and dry to touch and easily peel off the baking sheet. Cut and store in an airtight container.