Peel and core the apples, chop the red onions. Place both in a large pan with the sultanas, lemon zest, juice and vinegar. Bring to the boil and simmer gently for 25 minutes, stirring frequently. Add the sugar, bring to the boil again and stir until it has dissolved.
Simmer rapidly for 5 minutes, and stir frequently again otherwise the sugar will catch and burn. Chop the mint leaves finely and add to the mix, stir in and leave to cool for 5 minutes before bottling.