Dissolve the yeast in the warm milk and let stand for 5 minutes. Place the flour and salt in a food processor and make a well in the center. Add the butter and pour the milk mixture over. Using a dough hook, knead the dough on the “bread” setting for about 6 minutes. Add more milk or flour as needed. Form the dough into a ball, spray the same bowl with oil oil, cover loosely with plastic wrap, and let the dough rise in a warm place until nearly doubled in size. (I usually turn the oven on 350 degrees F for about 1 minute just until it’s a warm, turn it off, and let the dough rise in the oven).
To make the Onion/Bacon Mixture:
Fry up the bacon. When the bacon is done add the onions to it along with 2 tablespoons of butter. Reduce the heat and let the onions slowly caramelize to a nice golden brown, about 30-40 minutes. Remove from heat and allow the mixture to cool.
To make the Filling:
In a bowl, combine 1 ½ cups full fat sour cream, eggs, 2 tablespoons all-purpose flour, 2 teaspoons salt, 1 teaspoon caraway seeds, and some freshly ground pepper. Add the onion/bacon mixture to it and thoroughly combine everything.
If using refrigerated pie crust, grease the springform and gently press the crust into the pan, leaving about 1/2 inch or so margin along the top. If making the dough yourself, once the yeast dough has risen, butter a medium-sized springform (9 inches or so) and spread out the yeast dough on the bottom and up the sides (leaving about ½ inch from the top). The dough will keep retracting, just work fairly quickly and then immediately pour the onion mixture into it (it will hold the dough in place), sprinkle a few caraway seeds on top, and put it in the oven.
Place the Zwiebelkuchen on the middle shelf in the oven preheated to 400 degrees F and bake for 55-60 minutes, until the top is light brown and the center of the pie feels fairly firm to the touch. Let cool for at least 20 minutes before serving. Can be eaten lukewarm or cold.