Dry-fry the nuts in a nonstick pan for 3–4 minutes, stirring until lightly browned. Leave to cool slightly then roughly chop and mix with the sugar and spice.
Unfold the pastry and cut it into rectangles the same size as the base of an 18 x 28 cm (7 x 11 inch) small roasting tin. Wrap half the pastry in clingfilm so that it doesn't dry out. Brush each unwrapped sheet of pastry with melted butter then layer up in the roasting tin. Spoon in the nut mixture then unwrap and cover with the remaining pastry, buttering layers as you go.
Cut the pastry into 6 squares, then cut each square into 4 triangles. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 30–35 minutes, covering with foil after 20 minutes to prevent it overbrowning.
Meanwhile, make the syrup. Pare the rind off the citrus fruits with a zester or vegetable peeler then cut the rind into strips. Squeeze the juice. Put the strips and juice in a saucepan with the sugar, cinnamon and water. Heat gently until the sugar dissolves then simmer for 5 minutes without stirring.
Pour the hot syrup over the pastry as soon as it comes out of the oven. Leave to cool, then chill for 3 hours. Remove from the tin and arrange the pieces on a serving plate, sprinkled with slivers of pistachio. Store in the refrigerator for up to 2 days.