Preheat the oven to 170°C/325°F/Gas mark 3. Grease a 23cm/9in tart tin.
Using a large mortar and pestle, pound the almonds with the lemon zest. Add the remaining ingredients gradually, pounding together to form a thick paste. Spread the paste evenly into the tart tin, and bake in the oven for 1 hour or until the cake is golden on top. Cool on a wire rack.