Combine the flour, baking soda and salt in a large mixing bowl, and stir everything together. Make sure the batter is light and free of clumps. Set aside.

In a separate large bowl, mix together the sugar and olive oil until they are pale and well combined. Add the eggs gradually, mixing well. Then add the lemon zest, rosemary, and vanilla essence, stirring until incorporated.

Using a spatula, gently fold the dry ingredients into the wet mixture, mixing just enough to eliminate any remaining flour streaks. Remember that overmixing may result in a dense sponge.

In the prepared baking tin and pour the batter inside. Cook in the oven for 30 to 35 minutes, or until a toothpick inserted in the middle comes out clean.

Remove the cake from the pan and let it cool fully on a wire rack for 10 minutes. Top with lemon slices or rosemary sprigs and serve.