Place whole unpeeled oranges in a large saucepan or stockpot, and cover with cold water. Bring to a boil over high heat. Reduce heat, and simmer gently, 2 hours. Drain; set oranges aside to cool. (Navel oranges may have a few seeds. Halve the cooked oranges, and remove any strays before using the rest of the fruit.)

Preheat oven to 350 degrees. Butter and flour a 9-inch springform pan; set aside. Halve cooked oranges, and remove any seeds. Place 7 halves in a food processor, and pulse until pureed but still chunky. This should yield about 3 cups; set aside.

In a small bowl, whisk together the ground almonds, flour, baking powder, and salt; set aside. In the bowl of an electric mixer fitted with the whisk attachment, beat the eggs with 1 cup sugar on medium speed until light and fluffy. Stir in orange puree until just combined. Stir in flour mixture; pour into prepared pan.

Bake until cake is golden brown and a cake tester inserted in the center comes out clean, about 1 hour. Transfer pan to a wire rack. After 15 minutes, run a small spatula or paring knife around edge to loosen cake; let cool completely.

Meanwhile, make orange topping: Chop remaining 5 orange halves into 1/2- inch pieces, and set aside. In a medium saucepan, combine remaining cup sugar with 3/4 cup water. Bring mixture to a boil, stirring until sugar has dissolved.

Add chopped oranges to saucepan, and reduce heat to medium. Simmer gently until most of the liquid has evaporated and thickened into a syrup, about 15 minutes. Remove from heat, and set aside to cool completely.

To assemble, remove ring from pan; transfer the cooled cake to a serving platter. Arrange chopped oranges and any remaining syrup over top of cake. Cut into wedges, and serve.