1. Finely grate the lime zest from all four limes into a mixing bowl and add the evaporated milk. Using an electric mixer, whisk the evaporated milk on high speed until thick and double in volume.
2. Cut the limes in half and squeeze out the juice into a small saucepan.
3. Split the vanilla pod lengthways using a sharp knife. Scrape out the black seeds from the centre and add to the pan, along with the sugar.
4. Heat gently, stirring until the sugar has dissolved. Whisk the hot syrup into the milk until fully combined.
5. Carefully fold in the fromage frais and quark and pour into a plastic freezer container. Cover and freeze for 4?5 hours until firm.
6. Remove from the freezer 10 minutes before serving. Serve 2?3 scoops per serving sprinkled with the crushed ginger biscuits.