1 Preheat the oven to 350ºF. Set a rack in the middle position of the oven. Brush the bottom and sides of a 9×2 inch cake pan with olive oil.
Line the bottom of the pan with a circle of parchment paper. Brush the parchment with oil.
Generously sprinkle the pan with 2 to 3 tablespoons of sugar, tilting it to coat the bottom and sides, and tap out the excess.
2 Assemble the dry ingredients: In a bowl, whisk together the flour, baking powder, and salt together to combine.
3 Mix the batter: In a separate large bowl, rub the orange and lemon zest into the remaining 1 cup of sugar with your fingers until the sugar is permeated with the zest.
Add the eggs and whisk for about 45 seconds, or until the mixture lightens in color. Stir in the olive oil and sherry, and whisk until smooth.
Add the dry ingredients to the batter and whisk just until smooth.
4 Prepare to bake: Set the cake pan on a baking sheet and pour the batter into the pan.
5 Bake the cake: Bake for 35 to 40 minutes, or until the top is golden brown and a toothpick inserted into the center of the cake comes out clean.
Place the pan on a rack to cool for 15 minutes.
Run a knife around the edge of the pan, and remove the sides and bottom (or invert the cake onto a plate if using a layer cake pan and then turn right side up). Remove the parchment and let the cake rest on the rack until it is cool.
6 Make the candied orange slices (optional): In a 9- or 10-inch deep skillet or saucepan over medium-high heat, bring the sugar, water, and lemon juice to a boil. Cook, stirring occasionally, until the sugar dissolves.
Add the orange slices and adjust the heat to a simmer. Simmer, turning the oranges with tongs in the syrup occasionally, for 15 to 18 minutes, or until they are translucent. (If the syrup reduces to a sticky thickness, add more water, a few tablespoons at a time.) Leave the oranges in the syrup until lukewarm.
7 Decorate the cake: Brush the top of the cake with some of the orange syrup. Place the candied slices on top and sprinkle with pistachios.