This is truly an amazing cake.  Though a little butter is used to prepare the pan, all the fat in the cake comes from the ground walnuts and the eggs.  The whole orange is brought to a boil and simmered slowly, then pureed to create a very moist batter.  The bitter rind of the orange is balanced with the sugar and this cake contains a lot of beneficial nutrients.  It's a bit of a challenge, but worth the effort  

(Prepare a 9 inch springform pan by cutting a circle of parchment paper to fit the bottom and lightly greasing it.)

Ingredients:
1 large orange
2 cups of water
5 eggs, separated
1 cup of sugar
1 Teaspoon rum or rum extract
8 oz of walnuts,
  ( ground fairly fine in processor)
Confectioners sugar or whipped cream, if desired

     Preheat an oven to 350 degrees.     
     In a sauce pan over medium heat, combine the orange with water and bring to a boil.  Reduce the heat to low and cook, uncovered, until only about 1/2 cup of liquid (which will become a little syrupy remains).  This will take about an hour.  Remove from the heat.
     When cool enough to handle, remove the orange, cut in half and remove and discard the seeds.  Put the orange halves with peels  and 1/4 cup of the syrupy liquid in blender or food processor and puree until smooth.
     Beat the egg yolks and sugar until smooth.  Then add the rum, orange puree, and ground walnuts and mix well.
     In a second bowl, beat the egg whites until stiff peaks form.  Stir about one-fourth of the whites into the yolk mixture to lighten it.   Then gently fold in the remaining whites just until no white streaks remain; do not overmix.  Pure into prepared pan. 
     Bake until set, about 1 hour.  Remove from the oven and let cool completely in the pan.  Release and remove the pan sides and transfer the cake to a serving plate.  Sift confections sugar over the top, if desired. and I serve it topped with whipped cream.