Preheat oven to 350℉ (175℃).
Grease and flour two 8" cake pans.
Purée the spinach with the lemon juice, and set aside.
Beat eggs, sugar and salt in a stand mixer (or with a hand held mixer) until it increases in volume, turns a pale yellow, and makes ribbons in the batter when the beaters are lifted.
While beating the mixture, slowly pour in the oil.
Sift the flour and baking powder, and with a large spoon, fold it into the batter in thirds, clearing after each addition.
When all the flour has been added, the batter will be smooth, thick and heavy.
Add the spinach purée.Then add the lemon zest and mix until well combined.
Divide the green cake batter between the two prepared 8" cake pans.
Bake for about 30 minutes, or until a skewer comes out clean when inserted into the center of the cake. Allow to cool in pans for about 10 to 15 minutes, then carefully remove and place on cooling racks.