Peel the apples and pears, quarter them, remove the core and dice or grate them very, very finely. Squeeze the lemon and add to the fruit with the preserving sugar. Let the juice steep overnight.
The next day, finely chop the dried tarragon leaves and add to the pear and apple mixture.
Bring everything to a boil over high heat while stirring, then cook over medium heat for 4 minutes.
Make a gel test. Pour the jam into jars rinsed with hot water with twist-off lids and close immediately.