Prepare the yeast dough by mixing the milk and sugar, and crumbling in the yeast. Let the mixture stand for about 15 minutes.
Meanwhile, sift the flour and salt into a bowl. Add the yeast mixture, water, and oil, and knead everything together with a dough hook until smooth.Cover the bowl and let the dough rise in a warm, draft-free place for about 1 hour.
About 15 minutes before the end of the rising time, peel the apples and grate them using a kitchen grater.
Finely chop or cut the onions into thin strips.
If using bacon, chop it into small pieces and fry it in a frying pan. Remove it and set it aside. In the same fat, sauté the onions. If making your onion tart without bacon, you can use other fat, such as butter or vegetable oil.
Season to taste with salt, pepper, and a little nutmeg. If you like, add some caraway seeds now. This will make the onion tart more digestible.
Knead the yeast dough briefly and roll it out so that it fits on the baking sheet. Press the corners down well. Cover and let it rise for another 20 minutes.
Meanwhile, preheat the oven to 425°F (220°C) with the rack in the middle.
Then spread the apple and onion mixture on top of the dough, and sprinkle with the chopped or dried marjoram.
In a bowl, mix the milk and egg yolks. Season it only slightly, as the apple, bacon, and onion topping is already very savory.
Now pour the liquid over the topping and bake in the oven for about 25 minutes.
Remove from the oven, cut into pieces and serve hot.