Put 2 large oranges into a saucepan and cover with water. Bring to a boil and simmer gently for an hour.
Drain and leave to cool. When cool, roughly chop the whole oranges, including the peel, but discarding any pips,
and blend to a creamy paste with 6 eggs in a food processor. Mix 250 g ground almonds, 250 g granulated sugar
and a tsp baking powder in a large bowl, then whisk in the orange-egg mixture. Pour the batter into a greased
and floured springform tin and bake in a 190°C oven for 45 minutes to an hour. Check it is cooked by inserting
a sharp knife, which should come out clean. When cool, remove from the tin, dust with caster sugar and serve
with a dollop of crème fraîche.