Preheat the oven to 180 °C.
Rinse and grate the lemon. Combine all the base ingredients and work into a dough.
Grease a rectangular baking tin (about 24 x 30 cm) and line with lightly greased baking parchment. Press the mixture into the cake tin. Bake in the lower part of the oven for 10-13 minutes. Leave to cool slightly.
In the meantime, press the thawed blueberries through a sieve to give you just the blueberry juice. Stir in the lemon juice.
Combine the sugar and flour in a bowl. Whisk in the eggs. Finally add the blueberry juice and whisk to a smooth mixture. Pour the filling over the base and bake at the bottom of the oven for a further 15 minutes. Leave to cool and then cut the cake into small pieces and sprinkle lightly with icing sugar.
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