Preheat your oven to 350°F (175°C) and arrange an oven rack in the center.

Generously coat the bottom and sides of a 9-inch springform pan with olive oil, then line the bottom with parchment paper and coat the paper with oil.

Zest one blood orange to yield 1 tablespoon of zest and set aside.

Trim about an inch off each end of 5 blood oranges, then carefully cut off the peel and white pith following the curve of the orange.

Slice the peeled oranges into 1/4-inch rounds and arrange them in overlapping concentric circles on the bottom of the prepared pan, filling any gaps with smaller pieces.

Sprinkle the arranged orange slices with 1 tablespoon of granulated sugar and place the pan on a baking sheet; set aside.

In a medium bowl, whisk together the flour, baking powder, and 1/2 teaspoon kosher salt.

In a separate small bowl, stir together the 1/4 cup blood orange juice, 1/2 cup Campari, and reserved orange zest.

In a large bowl, using a hand mixer or stand mixer with a whisk attachment, beat the 3 large eggs and 1 cup granulated sugar until light, thick, and fluffy, about 5 minutes.

Gradually add 1 cup extra-virgin olive oil in a slow stream, beating for another 2 minutes until fully incorporated.

Add the Campari-blood orange juice mixture and whole milk to the egg mixture, mixing until just incorporated.

Gradually add the dry flour mixture to the wet ingredients, beating on low speed only until just combined and no streaks of flour remain.

Pour the batter into the prepared springform pan over the orange slices.

Transfer the baking sheet with the cake to the preheated oven and bake for about 1 hour, or until the center is firm to the touch and a toothpick inserted into the center comes out clean.

While the cake bakes, prepare the syrup: Combine 1 cup blood orange juice, 2 tablespoons granulated sugar, and a pinch of kosher salt in a small saucepan.

Bring the syrup mixture to a boil over medium heat, stirring occasionally, and cook until it reduces to about 1/2 cup and is slightly thickened, about 15 minutes (watch closely to prevent boiling over).

Remove the syrup from the heat, stir in 2 tablespoons of Campari, and allow it to cool.

Let the baked cake cool in the pan for about 30 minutes, then run a knife around the edges of the pan before carefully removing the outer ring.

Invert the cake onto a serving plate and gently peel away the parchment paper to reveal the orange topping.

Drizzle the cooled blood orange Campari syrup generously over the cake before serving.