Bring apple juice and elderflower to the boil and let it cool down. Then cover and leave to cool for at least 2 days.
Pour the elderberry and apple mixture through a fine sieve. Measure out 750 ml of juice (if necessary fill up to 750 ml with apple juice). Mix with jam sugar and lemon juice in a large pot and bring to the boil while stirring.
Let it boil for about 4 minutes while stirring constantly.
Wash the mint and shake dry. Pluck the leaves from the stems and place them in the prepared twist-off jars. Pour in the jelly, close immediately and turn upside down for about 5 minutes.
Turn glasses over and let them cool down. Shelf life approx. 6 months.