Preheat your oven to 400°F / 200°C.
Heat the olive oil in a large, oven-safe skillet over medium-high heat.
Add the butternut squash, season with salt and pepper, and cook for 4 minutes, stirring occasionally, until it begins to soften.
Reduce the heat to medium, add the sliced leek, and cook for another 4 minutes.
Add the kale and spinach to the skillet in batches.
Cook, stirring frequently, for 3 to 5 minutes until all the greens are completely wilted.
In a separate bowl, whisk the eggs with the lemon zest and a pinch of salt and pepper.
Pour the egg mixture over the wilted greens in the skillet.
Reduce the heat to medium-low and cook for 2 minutes, using a spatula to gently move the vegetables around so the egg is evenly distributed and just begins to set at the bottom.
Sprinkle the crumbled feta over the top of the kuku.
Transfer the entire skillet to the preheated oven and bake for 8 to 10 minutes, or until the eggs are fully set and just cooked through. Carefully remove from the oven and let it cool for about 10 minutes before sliding it onto a platter.
Garnish with fresh cilantro, cut into squares, and serve warm or at room temperature.
After wilting the spinach and kale, squeeze out any excess water before adding the eggs. This is the key to preventing a soggy kuku.
Keep a close eye on the kuku in the oven. Pull it out when the center is just set (a slight wobble is okay) to ensure a tender, not rubbery, texture.
