Zest your orange, then remove and discard the skin and pith. Cut the remaining flesh up into chunks. Put the zest and flesh into a food processor with the margarine and brown sugar. Blitz until you have a smooth paste, then add the eggs and blitz again till you have a weird-looking, separated liquid. Pour this out into a mixing bowl and sieve the flour, baking powder and xanthan gum over the top. Add the ground almonds. Using a wooden spoon, mix it thoroughly together, then pour the batter into your tin and
When the cake is completely cold, take it out of the tin and place on your serving plate. In a small bowl, mix your icing sugar and lemon juice to make a smooth, slightly runny icing. Drizzle the icing over the cake and, if you like, pop some flaked almonds and mixed peel on top.