Position a rack in the center of your oven and preheat to 350º.
As the oven preheats, place the butter in a 9" round cake pan and put it in the oven to melt. Sprinkle the brown sugar evenly over the butter and return the pan to the oven for about 5 minutes until the sugar is moistened and distributed evenly over the bottom of the pan.
Meanwhile, use a sharp paring knife to slice the top and bottom from the blood oranges. Place a cut-side down, and, following the curve of the orange, cut away the peel and white pith. Cut the orange cross-wise into 3/8" rounds. Repeat with the remaining oranges. Reserve any juices to use in the cake (I like to squeeze the butts and peels to get every last bit of juice).
Lay the orange rounds over the buttery sugar in the pan in a single layer using the fattest pieces and starting with the outer ring, and filling in the center with the smaller pieces. Set aside while you make the cake batter.
Make the cake batter:
In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter, sugar, zest and ginger until fluffy and lightened in color, about 5 minutes on medium speed, scraping down the sides of the bowl occasionally. Add the eggs one at a time, beating after each addition until incorporated, scraping down the sides of the bowl as needed.
Sift the flour, baking powder and salt into a medium bowl.
In a measuring cup, stir together thecrème fraîche, blood orange juice and vanilla extract.
With the mixer on low, add 1/3 of the dry ingredients to the butter mixture, let stir to incorporate, then scrape down the sides of the bowl. Add half of thecrème fraîche mixture, let incorporate, then scrape. Repeat, adding another 1/3 of the dries, half of thecrème fraîche, and the rest of the dries. Remove the bowl from the mixer and fold the batter with a rubber spatula to make sure the batter is completely homogeneous.
Dollop the batter over the orange slices, which may have released some juices and that's a-ok, and spread it evenly.
Bake the cake until a toothpick inserted in the center comes out clean, 35-45 minutes. Let the cake cool for 10 minutes. (If the cake cools too much, the orange goo may stick the pan; no worries, just return the cake to a hot oven for 5 minutes or so to re-melt it.)
Loosen the edges of the cake with a thin knife or offset spatula, invert a large plate over the top of the cake. Wearing oven mitts, grasp the cake and plate together and bravely flip them both over. Rap the plate on the counter a few times to dislodge, then remove the cake pan. Let cool to room temperature, about 1 hour – the cake is still baking from residual heat, so try to resist cutting into it too soon.
For the cleanest slices, use a serrated bread knife to gently saw through the orange slices, which can be a bit messy. Serve slices with extracrème fraîche, or crème anglaise (see headnote).
The cake is best served shortly after cooling, but will keep for up to 3 days at room temperature.
Inspired by the vegan Ginger-Clementine Upside-Down Cake in issue 24 of Jamie magazine.I used the zest of 2 small satsuma mandarins in this cake; if you prefer to use blood orange zest, zest 1-2 blood oranges before cutting off the peels to make the topping. You can make your own crème fraîche by stirring 1 tablespoon buttermilk into 1 cup of heavy cream and letting it sit in a warm spot for 24 hours; alternately, you can substitute the same amount of whole milk in the recipe and increase the flour to 1 3/4 cups – these two variations taste almost identical.The cake benefits from a creamy accoutrement; serve slices with additional crème fraîche, or decorate a pool of crème anglaise with drips of blood orange reduction.All ounce measurements are by weight.Nutritional values are based on one of eight servings.