Mix the flour, eggs and milk with the mixer until smooth. Season with salt and pepper. Let the dough swell for about 10 minutes, then mix the mineral water into the dough.
Fry the diced onion in oil until translucent, add the vegetables and season. Dice the feta cheese.
Line the drip pan or baking sheet with parchment paper. Pour the batter on top and spread the vegetables and cheese on top. Bake at 150 ° C for 25-30 minutes until golden brown.