Mix the crushed biscuits with the melted butter in a bowl.
Press the crumbs into the base and up the sides of a 20x30cm rectangular loose-bottomed tart tin or 23cm (9in) round loose-bottomed tart tin. Chill for 10 minutes.
Whisk together the condensed milk and cream, then add the lime juice and zest (watch it magically thicken!). Pour into the tart case and chill or at least 1-2 hours.
Remove the pie from the tin and place on a serving plate. Cut into slices and serve with the crème fraîche and lime zest.
This recipe can also be made with lemons – replace the 5 limes with 4 lemons.
Recipe courtesy of Carnation