Preheat your oven to 190
Add the butter to a frying pan over medium heat. Once melted add in the thinly sliced onion and cumin seeds and cook until onions are soft.
Add in the kale and spinach in handfuls until everything's wilted down. Add the lemon juice, turn up the heat and pour in the vegetable stock, a pinch of salt and pepper, and cook until the liquid has evaporated.
Score a 1cm border around each puff pastry rectangle. Place on a baking tray lined with parchment paper and evenly divide the spinach mixture between the two and create as large a well in the middle of each as possible for the eggs.
Crack one egg into each and, optionally, crumble some feta on top. It's important to use eggs that are as fresh as possible because if the white is too liquidy it'll easily spread out all over the baking tray. You can always crack the egg into a bowl first if you're uncertain.
Sprinkle a little salt and pepper over each and place in the oven to bake for about 15 minutes or until the whites of the egg are set.
Mix the za'atar and oil together and drizzle on top of the baked tarts and serve.