In a medium saucepan, place oranges and add enough water to submerge the oranges.
Cover the saucepan with a lid and bring to a boil.
Once the water boils, lower the heat and keep simmering for 2 hours.
Preheat your oven to 180°C (350°F).
Grease an 8-inch baking pan with butter and dust with some flour.
Slice the oranges into quarters and remove the bitter part in the middle and the main eye in the center.
Place the sliced oranges in a food processor and pulse until you get a smooth puree.
Add almond meal and sugar. Mix until thoroughly combined.
Add eggs and baking powder. Mix until you get a smooth batter.
Pour cake batter into the prepared baking pan.
Use a spatula to even out the top of the flourless orange cake batter.
Sprinkle sliced almonds on top of the mixture.
Place the baking pan in the lower part of the oven for 55 to 60 minutes.
Let it cool for about 30 minutes, then remove the flourless orange cake from the pan.
Dust with powdered sugar or add more sliced almonds on top.
Your orange and almond cake is now ready for serving!