Heat the oven to 350°F (325 with fan) and set an oven rack in the center position. Grease the bottom and sides of 9-inch round cake pan. Line the bottom of the pan with a round of parchment paper and coat it with more butter. Set the pan aside.

Finely grate zest from one orange (about 2 teaspoons), and set zest aside. Slice off the tops of the oranges to expose the fruit and use a sharp knife, or even better, a mandolin, to slice oranges into very thin (1/8-inch) rounds. Remove and discard any seeds and repeat with all the oranges (you should have about 15 or 20 slices total).

Squeeze 1 tablespoon of juice from the reserved ends of the oranges and add to a small saucepan. Add 3 tablespoons (45 grams) butter, and the brown sugar. Turn heat to medium and stir until sugar melts, about 3 minutes. Scrape mixture into prepared pan and tilt in all directions to spread across the parchment. Arrange the orange slices in an overlapping pattern across the bottom of the pan and set aside.

In a large bowl, whisk together the almond flour, all-purpose flour, baking powder and salt. In a stand mixer fitted with the paddle attachment (or in a large bowl if using a hand mixer),  cream together the remaining 2 sticks (225 grams) butter with granulated sugar until light and fluffy, about 3 to 4 minutes. Scrape down sides and bottom of bowl as needed. Beat in eggs, one a time, letting each fully incorporate before adding the next. Add sour cream, reserved orange zest and lemon zest, lemon juice and vanilla and beat until fully mixed. Gently fold in the dry mixture by hand, scraping the bottom and sides of the bowl to make sure all the flour is incorporated. The batter might look a little curdled from the addition of lemon juice - this is normal!

Gently scrape the batter over the blood orange slices, being careful not to shift them, and smooth the top. Place the cake on a baking sheet to catch any drips and transfer to the oven. Bake until cake is golden brown and a toothpick inserted in the center emerges clean, 40 to 50 minutes. Transfer the pan to a wire rack and let the cake cool for 10 minutes. Run a thin knife around the edges of the pan to loosen it then invert onto a platter and remove parchment. Allow to cool completely before serving.

If you want a super shiny top, add apricot jam to a small bowl and mix with 1 teaspoon hot water to loosen. Brush the glaze over the oranges. Slice and serve with whipped cream or yogurt.