Combine the apples, lemon juice and zest in a large saucepan. Cover with 1 litre of water, bring to the boil over a medium heat, then reduce the heat and simmer for 1 hour.
Transfer the mixture to a muslin-lined fine sieve and strain into a bowl, discarding the pulp. This should yield about 2 cups of liquid.
Combine the liquid and sugar in a large saucepan and bring to the boil, stirring to dissolve the sugar and cook for 15 to 20 minutes. Test whether the jelly is set. If not, return to the heat and test after 5 minutes.
Remove from the heat, stand for 10 minutes, then skim the film from the surface. Add the rosemary and ladle into clean, dry jars. Seal the lids, invert jars for 2 minutes, then turn upright and cool completely. Store in the refrigerator for up to 3 weeks.