Mix eggs, flour and milk into a smooth dough and season well with salt and pepper. Let it soak for about 15 minutes. Preheat the oven to 180 ° C (fan, gas: level 2).
In the meantime, finely dice the onions and chop the garlic. Heat the oil in a pan and sweat the onion with the garlic until translucent. Drain the spinach well (it is best to squeeze the frozen spinach a little) and fry it in the pan until it has collapsed a little. Season with salt, pepper and nutmeg.
Dice the tomatoes and feta cheese. Stir the mineral water into the batter. Don`t worry that the dough is so soft, that`s how it should be. It becomes firm when it is baked.
Line the oven pan with parchment paper and fill in the batter evenly. Top with tomatoes, spinach and cheese. Season if you like (I like to add some chilli for the right heat). Put in the oven and bake for about 20-25 minutes, until the edge is brown.
You can of course top the dough with other delicacies (e.g. bacon, ham, minced meat for those who do not want to do without meat).