Preheat the oven to 350°F. Grease and line a 9-inch springform pan or cake pan.
Rinse the orange well, then place it in a pot covered with water. Bring to a boil and simmer for 30 minutes. Drain, then transfer the whole orange to a blender and purée until mostly smooth (some small pieces are fine).
In a large mixing bowl, combine the almond flour, ground pistachios, honey, browned butter, eggs, baking soda, and vanilla. Stir in the orange purée until fully mixed.
Pour the batter into the prepared pan and bake for 50–55 minutes, or until a toothpick inserted in the center comes out clean. If the cake browns too quickly, tent loosely with foil.
Cool for 10 minutes before gently removing from the pan.
While the cake bakes, prepare the caramel: in a small pot, heat orange juice, butter, and honey (or brown sugar) over medium heat. Simmer until reduced to a thick glaze.
While the cake is still warm, spoon the sticky orange caramel over the top.
Garnish with pistachios, almonds, or a dollop of mascarpone or ice cream before serving.