• Preheat oven to 350ºF | 180ºC and grease a cake pan with olive oil.
• In a large bowl, combine flour with baking powder and set aside.
• Separate the egg yolks from the whites, beat the egg whites and set aside.
• Put the watercress, oil, sugar and egg yolks in a blender and mix well until you get a green paste.
• Mix the previous preparation with the flour and beat with an electric mixer.
• Add the egg whites and gently fold the dough with a spatula until the egg whites disappear completely.
• Bake for 35-40 minutes.
• Remove from the oven and cool on a wire rack. Drizzle over the glaze, making zigzag lines and let them drain freely. Top with chopped walnuts.
• Transfer to a plate and it's ready to serve!

Topping (optional)

• Place the icing sugar on a bowl. Add the lemon juice and beat vigorously with a spatula for 10 seconds, until smooth. Add more lemon juice, if needed.