Beat the butter and sugar together in a mixing bowl until pale and creamy. Gradually mix in alternate spoonfuls of beaten egg and polenta. Stir in the ground almonds and baking powder then mix in the lemon rind and juice.
Spoon the mixture into a greased 18 x 28 cm (7 x 11 inch) roasting tin. Sprinkle the canned cherries over the top then the flaked almonds.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 25–30 minutes until well risen, the cake is golden and springs back when gently pressed with a fingertip.
Leave to cool in the tin, dust with sifted icing sugar then cut into 14 bars and lift out of the tin. Store in an airtight tin for up to 2 days.