In the bowl of an electric mixer fitted with the dough hook, combine the yeast, sugar, and half the milk. Let the mixture sit for about 5 minutes for the yeast to bloom.

Add the remaining milk, flour, butter, and salt, and mix on medium-low to medium speed until the dough forms into a smooth ball, about 2 minutes. Cover the bowl with plastic wrap or a tea towel and set it in a slightly warm, draft-free place to rise until nearly doubled in size, about 1 ½ to 2 hours.

Add the bacon to a cold skillet and place it over medium heat. Cook until it renders most of its fat and becomes crispy, about 8 to 10 minutes. Remove the cooked bacon to a bowl with a slotted spoon, leaving the bacon grease in the skillet.

If there’s a lot of bacon grease left in the skillet, remove some so you have about 2 tablespoons remaining to cook with. If there isn’t a lot, add some butter so you have about 2 tablespoons of fat in the pan.

In the skillet of bacon grease (and, if needed, butter), add the onions. Cook over medium heat until soft and translucent but not browned, about 10 to 12 minutes. Stir in the flour and then set aside to cool.

In a mixing bowl, whisk together the eggs, heavy cream or sour cream, and the seasonings. Stir in the cooked onion and flour mixture, and most of the cooked bacon (reserve about 1 tablespoon for the top).

Preheat the oven with the rack in the middle to 400°F (200°C).

Punch down the dough to deflate it and transfer to a 9-inch springform pan. Evenly press the dough into the bottom and up the sides of the springform pan. Use your fist to help push the dough up against the edges.

Pour the filling into the prepared crust. Sprinkle the reserved crispy bacon evenly over the top. Bake for 35 to 40 minutes until the crust is golden brown and the filling is set.

Remove from the oven and allow it to cool for about 10 minutes before removing from the pan and serving. Zwiebelkuchen is best served warm but can also be eaten at room temperature or cold. For the most authentic experience, serve with chilled Federweißer or Riesling wine, or even a German lager if that’s your preferred libation.