1. Heat your oven to 180C/Gas Mark 4.
2. Melt the butter, then crumb the digestive biscuits in a food processor, or by putting them in a large bowl and crushing with the end of a rolling pin.
3. Mix together the digestives, melted butter and sugar, then press into the bottom and sides of a 20 cm springform tin. Bake for 15 minutes, then leave to cool. Turn your oven temperature down to 160C/Gas Mark 3.
4. In a large bowl, whisk together the egg and egg yolks until glossy then whisk in the condensed milk.
5. Zest and juice your oranges and lemons, then mix all the zest together in one bowl and all the juice together in another bowl or jug. Remove the teaspoon of zest and the tablespoon of juice you will need for your topping and keep to one side. Whisk the rest in with the filling. Pour on top of the biscuit base and cook in the oven for 20 minutes.
6. Remove and leave to cool, then chill overnight in the fridge (if making in the morning, it should chill within 5 hours if required for the same day).
7. To make the topping, whip the cream, crème fraiche, remaining juice and icing sugar together until thick. Spoon onto the pie and spread, then sprinkle the remaining zest over the top.