Place green apple cores in a food processor and whiz until smooth. Place in a saucepan with apple cider and caster sugar, plus apple cider vinegar and lemon juice to taste.
Cook, stirring to dissolve the sugar, until mixture reaches 118 decrees C on a sugar thermometer.
Pour into a sterilised heatproof container. The paste will be deep orange and will set at room temperature. Serve on your cheeseboard.