Heat oven to 180C/160C fan/gas 4.
Line the base and sides of a shallow
rectangular tin (about 23 x 33cm) with
baking parchment, snipping the corners
so that it comes up the sides of the tin.
Tip the flour, sugar and butter into a
food processor and whizz until it forms fine crumbs. Tip into the prepared tin
and smoothly level with the back of a
metal spoon, pressing it down lightly.
Bake for 20-25 mins until pale golden.
To make the topping, finely grate the
zest from the lemons and orange and
squeeze out all the juice. Measure the
juice – you need 120ml. Whisk together
the eggs and sugar using an electric
whisk for 1 min, then add the citrus
zest and juices, and whisk again briefly.
Sift in the flour and whisk well to mix.
Pour over the shortbread and bake for
15-20 mins until the topping has just
set. Cool completely in the tin, then
lift out of the tin using the lining paper.
Dust thickly with icing sugar and cut
into 3 down the length and 6 across
to make 18 bars.