Preheat oven to 200 C, Gas 6 . Line a large rectangular or square oven-proof pan with parchment paper, then drizzle a little olive oil onto the parchment and rub it over the base. (Alternatively, use 2 smaller containers, or 2 muffin tins for 16 individual kuku.)
In a large mixing bowl, combine eggs, zucchini, potatoes, green onions, saffron, yogurt, flour, baking powder and salt. Season with black pepper and mix until evenly combined.
Pour mixture into prepared pan, ensuring that mixture is at least 2 cm clear of the rim so it does not overflow during cooking. Bake in preheated oven for 35 minutes (or 25 to 28 minutes for smaller or individual kuku) or until top of kuku is golden and begins to brown.
To check if it is cooked, insert a knife into center of kuku; if it comes out clean of liquid (raw egg, in this case) but moist, the dish is done. If it looks wet and eggy, it will need a few more minutes.
Allow to cool slightly, then flip the baking dish upside-down and tip out the kuku onto a chopping board. Peel off parchment paper and cut kuku into pieces to serve.