Mix ricotta and crème fraîche in a food processor, then add icing sugar, lime zest and juice, and ginger syrup. Squeeze excess water from gelatine, place gelatine in a cup and microwave for 10 seconds to dissolve (it must not boil). Combine gelatine with ricotta mixture, then remove food processor blade and stir in chopped ginger. Place in a plastic container (approximately 12cm × 16cm) lined with baking paper and refrigerate for a few hours until firm. For the granita, dissolve the sugar in boiling water, then add the lime zest and leaves. Steep for 5 minutes, then add the lime juice. Remove the leaves and freeze the syrup in a shallow container. Stir and scrape with a fork after 40 minutes and every 30 minutes thereafter to make a fluffy granita. Crush the gingernuts in a food processor or with a mortar and pestle and set aside. Remove cheesecake from container and cut into long slices. Serve cheesecake sitting on a bed of gingernut crumbs with granita and a little chopped ginger in syrup alongside, then grate a little lime zest over the top.