Put the whole oranges in a saucepan and cover with water. Bring to a boil, then reduce heat and simmer for 90 minutes, topping up water as needed, until a knife slides through the skin with no resistance.

Drain the oranges and let them cool for 15 minutes. Cut into quarters, remove any large seeds, and blend the quarters, peel included, in a food processor until smooth and thick.

Heat the oven to 180 C / 350 F. Grease a 23 cm / 9-inch springform pan with the oil and line the base with parchment.

In a large bowl, whisk the eggs and sugar together for 2 to 3 minutes, until pale and slightly thick.

Add the ground almonds, baking powder, and salt to the egg mixture. Fold in the orange puree until the batter is smooth and even.

Pour the batter into the pan and smooth the top. Bake for 45 to 50 minutes, until the top is golden brown and a skewer inserted in the center comes out clean.

While the cake bakes, combine the orange juice, sugar, water, and cinnamon stick in a small saucepan. Bring to a simmer and cook for 5 minutes, until slightly thickened, then remove the cinnamon stick.

Let the cake cool in the pan for 10 minutes. Poke holes over the top with a skewer, then pour the warm syrup slowly over the cake so it soaks in.

Scatter the toasted flaked almonds over the top. Let the cake cool completely in the pan, about 20 minutes, before slicing.