Orange, barberry and sumac cheesecake

Method

1. Heat the oven to 160°C fan/ gas 4. Grease and line the base of the cake tin with butter and baking paper. Whizz the biscuits in a food processor or put them in a freezer bag and bash with a rolling pin to form fine crumbs. Pulse or stir in the melted butter along with a pinch of salt, then press into the cake tin in an even layer. Bake for 10 minutes, then set aside (keep the oven on).
2. Meanwhile, put the barberries in a bowl. Finely zest both oranges and reserve the zest, then squeeze the juice from
   one of them over the barberries. Set aside the second orange to decorate the cake.
3. In a large bowl, use a balloon whisk to mix the cream cheese and sugar together until smooth. Gently beat in the eggs, one at
   a time, then add the sumac, reserved orange zest and a pinch of salt. In another bowl, whip the cream to soft peaks, then fold it into the cream cheese mixture along with the flour until smooth. Fold in two thirds of the barberries and all the orange juice they’re soaking in. Tip the mixture over the biscuit base, then scatter over most of the remaining barberries (reserving a few to decorate).
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4. Bake the cheesecake for 45 minutes, then turn the oven off and leave it inside (without opening the door!) for another hour. Remove from the oven to cool completely. Run a palette knife around the edge of the cheesecake to release it slightly – but leave it in the tin. Chill for at least 4 hours to firm up (see Make Ahead).
5. When ready to serve, remove the cheesecake from the tin and decorate it with the remaining orange cut into segments and scattered over, along with the reserved barberries and an extra sprinkling of sumac.