Put the flour in a bowl. Add the butter
and rub in with the fingertips until the mixture resembles coarse
crumbs. Stir in the ground almonds and caster sugar. Mix to form a
firm dough. Knead gently on a lightly floured surface. Wrap in
clingfilm (plastic wrap) and chill for 30 minutes.
Cut into quarters and roll out each piece to a 23 cm/9 in round. Place
on lightly greased baking (cookie) sheets and bake in a preheated oven
at 190°C/ 375°F/gas mark 5 for 15 minutes until a pale biscuit brown.
Cool on wire racks.
Meanwhile, put the granulated sugar and
water in a pan. Heat gently, stirring, until the sugar dissolves. Boil
rapidly for 2 minutes, then add the rind and juice of one of the
lemons, the chopped apple and the flaked almonds. Cook until thick.
Leave to cool.
When cold, place the first layer of
shortbread on a serving plate. Spread a third of the apple mixture on
top, then repeat the layers, finishing with a layer of shortbread.
Sift the icing sugar into a bowl. Add the rind and juice of the
remaining lemon and mix to a smooth paste. If necessary, add hot
water, a little at a time, to form a glossy, pouring consistency.
Drizzle over the shortbread and chill overnight before serving.