Squeeze the lemon. Wash apples, except 1, grate dry and cut in half. Sprinkle with 3 tbsp. lemon juice. Juice the apples in a juicer. 750 ml apple juice, measure (use the rest for other purposes). Mix with jam sugar, elderflower syrup and remaining lemon juice and bring to the boil. Cook for 4 minutes until bubbly
In the meantime, cut the remaining apple in half, remove the seeds and cut into small cubes. Cook the apple cubes and Prosseco the last minute. (Make jelly test!) Fill jelly into prepared glasses and close them immediately