Wash the oranges thoroughly and place them whole in a pot covered with water; bring to a boil and simmer for 1 hour until soft.

Drain the oranges and let them cool completely, then cut them into quarters, remove any seeds, and puree the fruit (skin and all) in a food processor until smooth.

Preheat your oven to 375°F (190°C) and grease a 9-inch springform cake pan, lining the bottom with parchment paper.

In a large mixing bowl, beat the eggs and 1 cup of sugar together until the mixture is thick and pale, about 3-5 minutes.

Gently fold in the orange puree, almond meal, breadcrumbs, and baking powder until the batter is well combined.

Pour the batter into the prepared cake pan and bake for 45 to 60 minutes, or until a skewer inserted into the center comes out clean and the top is golden brown.

While the cake bakes, make the syrup by combining 1/2 cup water, 1/2 cup sugar, and the cinnamon stick in a small saucepan; simmer for 5 minutes until the sugar dissolves and the liquid thickens slightly.

Allow the cake to cool in the pan for 10 minutes, then brush the warm syrup generously over the top of the cake.

Let the cake cool completely before removing it from the pan, then garnish with fresh orange slices before serving.