Wash the oranges thoroughly. Boil them for 1 1/2 hours or until they are extremely soft. This will remove the bitterness in the peel.
Allow the oranges to cool so that they can be handled, then open them and remove the seeds. (Make sure not to lose the juice–I like to work over the food processor bowl.) Then puree the oranges in a food processor or blender.
Beat eggs and sugar. Add ground almonds and mix well.
Add the orange puree. Mix well, then pour into a baking dish. (Some recipes say that this cake can be a little sticky so they recommend a cake pan with a removable bottom. But I just use a pie pan prepped with butter and flour, and I've never had a problem.)
Bake at 325 F for 1 hour. The cake will look dark on top but it's not burned.
Notes: I don't tend to have superfine sugar on hand, so I take regular sugar and give it a little spin in my electric spice grinder. It seems to work okay.
This cake is extremely moist and not overly sweet. The orange taste is really intense.
You can substitute almond flour for ground almonds. The flavor will be sweeter because almond flour has the bitter peel of the almond removed.
If you have stored almonds in the freezer (whether ground or flour), bring them to room temperature before baking.