Place the oranges in a large pot and fill with enough water to cover the oranges. Bring to a boil over high heat, then reduce to a simmer. Continue simmering for 2 hours. Then, drain the pot and let the oranges cool.

Preheat the oven to 350°F. Using a small, sharp knife, carve out the stem/stalk from the oranges. Make the same kind of shallow, small carving on the opposite end of the oranges as well (the bottom of the oranges).

Cut up the oranges (WITH the rind, seeds, membranes and all) into chunks and place in a food processor. Pulse until you've got a purée, and the only pieces of orange rind you see are the tiny specks; set aside.

In a large bowl, whisk the sugar and eggs together until combined. Add in the orange purée and whisk. Add the vanilla and whisk again to mix. To the wet mixture, add the almond meal and baking powder. Mix until it's all combined well.

Grease a 10 inch springform pan (or an equally deep pan) with baking spray (or softened butter). Pour the batter into your pan before sprinkling the top with sliced almonds. Bake the cake for approximately 1 hour and 10 minutes, checking on the cake at the 1 hour mark. Poke a toothpick in the center to check for readiness; it should come out with little to no crumbs.

Let the cake rest in the pan for 10 minutes before lifting the edges of the springform pan off the cake. Let it cool to room temperature, then dust the top with powdered sugar.