Heat oven to gas 4/180c/350 F Grease and line 20cm/8" deep cake tin (round) Cream margarine & sugar till light and fluffy then beat in egg yolks and lemon juice. Fold in flour baking powder and nuts. Whisk egg whites until stiff then fold into cake mixture with the grated beetroot. Pour into the tin and make slight dip in the middle. Bake for 1 1/4 - 1 1/2 hrs. Remove when skewer comes out clean and allow to cool slightly before turning onto a rack to cool completely. Beat butter, sugar and cheese till smooth. Halve cake and fill with 2 thirds of filling. Spread rest on top and sprinkle with icing sugar and nuts.