Preheat your oven to 350 degrees F. Grease a 9-inch round cake pan with olive oil or cooking spray, then line the bottom with a round of parchment paper and grease the parchment too. Set it aside.

In a medium bowl, whisk together the flour, baking powder, and salt. Set that bowl aside.

In a large bowl, whisk the sugar and eggs together for about 1 minute until the mixture looks slightly pale. Whisk in the olive oil, milk, and vanilla until smooth and fully combined.

Add the flour mixture to the wet ingredients and fold it in with a rubber spatula until just combined and the batter is smooth. Do not beat it. Fold in the minced rosemary and diced apple.

Pour the batter into the prepared pan and smooth the top. The batter is fairly thin and will spread evenly on its own.

Bake on the center rack for 40 to 45 minutes. The cake is done when the top is deep golden, the edges have pulled away from the pan slightly, and a toothpick inserted in the center comes out with just a few moist crumbs, not wet batter. Start checking at 40 minutes.

Cool in the pan on a wire rack for 15 minutes, then run a thin knife around the edge and invert the cake onto the rack. Peel off the parchment and let it cool completely right-side up before slicing. Dust with powdered sugar and serve with whipped cream if you like.