Preheat oven to 200C degrees. Sauté diced onion with a little oil on medium-high heat, until translucent and soft. Add chopped spinach and cook until soft. Remove from the heat and leave it to cool. In a large bowl, combine the eggs, flour and baking powder and stir until a smooth mixture. Optional, season with salt, black and cayenne pepper. Add milk, stir and add the spinach mixture. Mix again and pour into a greased baking pan or form quiche or pie (diameter about 24 cm). Sprinkle with crumbled feta cheese and bake 25-30 minutes or until center is hardened and the outer edge is golden brown. Let cool 5 minutes before serving. * Do not be a basis for slush, but the flour naturally gets thinner crust on the bottom and the sides of the form.