Preheat the oven to 165℃. Line a round cake tin with baking paper. If you crumple a piece of paper in your hand and then straighten it, it will be malleable and can easily be adjusted to the shape of the tin.
Sift the flour into a bowl, add baking powder, mix and set aside.
Wash the spinach, put it in a food processor, add lemon or orange juice and blend well.
Crack the eggs into the mixer bowl, add sugar and beat at high speed until the mixture becomes very light and very fluffy. It should increase volume significantly.
Continue beating, while very slowly adding the oil to combine everything nicely.
Add the blended spinach and mix at low speed, or use a spatula to evenly distribute the spinach in the batter.
Finally, add flour and baking powder and mix briefly with a spatula, but only until the ingredients are combined, no longer.
Pour the mixture into the cake tin, smooth it out and place in the preheated oven. Bake for about 40-45 minutes until a toothpick inserted comes out clean, with no wet pieces of cake stuck on.