Blood oranges look a lot like navel oranges from the outside, but don't be fooled. Beneath the peel lies a dark crimson flesh with tangy, citrusy notes and a distinct raspberry-like taste. Slice them thin and layer over a bed of rosemary and brown sugar that caramelizes as it bakes. The oranges are topped with a one-bowl olive oil cake batter. After cooling, it flips out perfectly for an elegant dessert topped with candied pistachios for color and crunch.
Preheat the oven to 350 degrees F. Spray a 9-inch round cake pan with nonstick cooking spray.
Combine the brown sugar, butter, rosemary and the finely grated zest of 1 blood orange (about 1 teaspoon) in a medium saucepan. Cook over medium heat, stirring constantly, until the butter is melted, the sugar has dissolved and the mixture comes to a full boil, 2 to 3 minutes. Remove from the heat and let cool slightly, about 10 minutes. Pour the mixture into the prepared pan and use an offset spatula to spread evenly (it's OK if the caramel looks grainy).
Finely grate the zest of 1 blood orange into a large bowl and set aside. Cut the ends off 3 of the blood oranges, then remove the skin in strips working to expose the flesh all the way around the fruit. Slice the oranges into 1/8-inch-thick rounds and remove any seeds. Arrange the slices over the brown sugar in the pan, overlapping them slightly as needed so that no sugar can be seen. Set aside.
Add 1 cup of the granulated sugar and the olive oil to the bowl with the orange zest and whisk to combine. Add the eggs and vanilla and whisk until lightened, about 4 minutes. Add the flour, baking powder and 1/2 teaspoon salt and mix until just combined, about 1 minute. Slowly pour in the milk and continue whisking until combined. Scrape down the sides of the bowl with a rubber spatula to make sure everything is incorporated.
Carefully pour the batter over the orange slices in the prepared pan and smooth the top. Bake until the cake is golden brown and a toothpick inserted into the center comes out clean, 40 to 50 minutes. Transfer to a rack to cool slightly, about 30 minutes. Run a small knife or offset spatula around the edge of the cake to loosen. Invert a large plate or serving platter on top of the pan and flip to unmold. Let cool completely, about 1 hour.
Meanwhile, line a baking sheet with parchment. Juice half of the remaining orange (save the other half for another use). Add the remaining 2 tablespoons sugar, 2 tablespoons of the blood orange juice and the remaining 1/4 teaspoon salt to a small saucepan and stir with a heatproof rubber spatula or wooden spoon to combine. Bring to a boil over medium-high heat, stirring occasionally, until the mixture turns rust colored around the edges, 2 to 3 minutes. Reduce the heat to low and fold in the pistachios until well coated and shiny. Pour onto the prepared baking sheet, breaking up any large clumps with a wooden spoon and allow to dry completely, about 15 minutes.
Chop or crush the candied pistachios with a heavy glass to break up any larger pieces. Sprinkle the pistachios around the rim of the cake before serving.
When measuring flour, we spoon it into a dry measuring cup and level off excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)