Clean the elderflowers well. Cut one lemon into slices.
Put the blossoms and lemon slices with apple juice, cinnamon and 250 ml of water in a well-chilled place for 24 hours. Sieve the next day.
Heat the jars in the oven at 150 °C convection oven.
Squeeze the remaining lemon and boil the juice with elderberry juice, wine, rum and jelling sugar while stirring for 4 minutes until bubbling.
Fill the hot jelly into the prepared jars and seal them immediately.