Edit: So whoever wants to give it a try, here's the recipe:
Dough:
375g flour (Type 405 in our case)
250ml milk
60g butter (room-temperature soft)
1/2 tablespoon sugar
1/2 teaspoon salt
1/2 cube fresh baking yeast (German cubes have 42g, so 21g)
Topping:
1kg onions, cut to fine rings
100g bacon, finely diced
if the bacon is lean, add 20g of your preferred fat (lard, ghee, sunflower oil, etc)
3 eggs
4 tablespoons Schmand (i think this will depend on where you are, but it's basically a sour heavy cream. Ours has 24% fat)
salt and pepper
ground nutmeg
1 tablespoon caraway seeds
Mix all the dough ingredients together, knead until you have a smooth dough. Let rest at room temperature until it has roughly doubled in volume (about 30 minutes)
Meanwhile, take a large(!) pan or pot, put in bacon (and extra fat if applicable) and lightly fry, then add the onions. It helps to separate the slices into rings while adding. Sautée until soft. Let them cool down.
Pre-heat your oven to 200°C top/bottom heat. Convection should work too, just reduce the temperature as always. Roll out your dough on a baking tray. We use baking parchment or whatever it's called in English. Pull up the sides a little so you have a small rim.
Mix Schmand (sour cream, whatever else you have as a substitute) and the eggs, add salt (quite a bit), pepper and nutmeg. Crush the caraway seeds if possible, add those too. Add the cooled-down onions and mix well. Spread evenly on top of the dough.
Bake at 200°C between 20 and 30 minutes until the bottom is done and the topping is no longer soggy (this will depend heavily on your oven. Just check regularly)
But beware, every town and sometimes family has their own, slightly different recipe
Also beware of the toxic farts brought on by this delicacy. I got thrown out of class as a kid once after having Zwiebelkuchen (same thing) the previous day.
😬💨