Heat the oven to 325°F. Prepare a 9-inch springform pan by greasing it and lining with parchment paper. Make the fennel sugar topping. Use a mortar and pestle or spice grinder to grind the fennel down to a coarse powder. In a small mixing bowl, use your hands to rub the ground fennel into the sugar and salt until the mixture is fragrant. Add in the 1½ teaspoons of olive oil and stir with a spoon until the mixture takes on the texture of wet sand. Set aside. Make the cake. Toss half the grapes with 2 teaspoons of flour and set aside. In a large mixing bowl, combine the olive oil, sugar, salt, baking powder, and eggs and whisk until well combined—the mixture should be thick and glossy. Add the sour cream and whisk until homogenous. Then add the flour and whisk until just combined before gently mixing in the floured grapes. Pour the batter into the prepared pan and place the remaining, un-floured grapes on top. Evenly distribute the fennel sugar over the batter. Bake for about 1 hour, or until golden brown and a toothpick inserted into the middle of the cake comes out clean. Let cool for 10 to 15 minutes before removing the cake from the pan. Serve warm or at room temperature. Note: Store leftover cake in an airtight container at room temperature for up to 3 days.