Method

1. For the filling: combine the juice of half the lemon with all the lime and orange juices. (Reserve the remainder of the lemon juice for the glaze.) Add the grated fruit zests and sultanas and stir to mix. Leave to soak until the sultanas have plumped up - about 1-2 hours.

2. For the almond pastry base: sieve the flour and salt into a mixing bowl, and stir in the ground almonds and sugar. Using your fingertips, lightly rub the butter into the flour mixture until the mixture resembles breadcrumbs.

3. Gradually add the beaten egg and knead the ingredients together. This mixture behaves more like a biscuit mixture than pastry dough, so it will remain quite crumbly even when the egg has been kneaded in. Cover and chill in the fridge for 30 minutes.

4. Preheat the oven to 200C/gas 6. Grease a 24cm flan tin, then press the chilled almond mixture evenly over the base and up the sides of the dish, leaving a little pastry overhanging the dish slightly. This mixture is almost impossible to roll out so it is best to press it into the dish with your fingertips, patching up any little holes as you go.

5. Put a piece of greaseproof paper over the pastry and cover with a layer of baking beans to weigh the paper down and keep the pastry in shape as it bakes. Bake in the oven for 10 minutes - until the pastry has just set. Take the flan out of the oven, remove the baking beans and paper, and return the dish to the oven for 5 minutes. Remove from the oven once again and trim round the top edge of the pie dish using a sharp knife to neatly cut away the overhanging pastry (this pastry burns easily, so this will help to neaten the edges and appearance of the finished pie). Reduce the oven temperature to 180C/gas 4.

6. For the filling: put the soaked sultanas and fruit juices in a pan and add the butter, ground almonds, walnuts and sugar and heat gently, stirring continuously, until the butter is melted and the ingredients are combined. Remove from the heat, stir in the eggs, and pour the mixture evenly into the pastry case. Bake in the oven for 15-20 minutes - until the filling is lightly set. Remove from the oven and leave to cool.

7. For the lemon glaze: pour the reserved lemon juice (from the remaining half lemon) into a small bowl. Sieve the icing sugar over the lemon juice and mix well to make a smooth, runny glaze.

8. Carefully remove the cold tart from the pie dish and place on a serving plate. Drizzle the lemon glaze over the tart and serve in slices - either on its own or with a dollop of crème fraîche.