Mash the potatoes throughly while warm. Use a potato ricer, if possible because there must be no lumps at all.Allow to cool.
Mix in eggs yolks, ground almonds and lemon juice and grated lemon peel. Beat the egg whites until nearly stiff, then add the castor sugar and beat again. Fold beaten egg whites into the potaot mixture. Pour into a lightly buttered 19cm-20cm round or square cake tin and bake for about 40 minutes, or until a toothpick inserted in the centre of the cake comes out dry.