In a bowl, mix the warm milk and sugar. Sprinkle the active dry yeast on top and let it sit for about 10 minutes until it’s frothy.

In a larger bowl, combine the activated yeast mixture with the flour, salt, and softened butter. Mix until it forms a dough, then knead on a floured surface until smooth. Place the dough in a lightly greased bowl, cover it with a cloth, and let it rise for about 1 hour in a warm area.

While the dough rises, heat 2 tablespoons of butter in a skillet over medium heat. Add the sliced onions and sauté until they are caramelized and golden brown, about 25 minutes.

In a separate pan, cook the chopped bacon over medium heat until crispy. Once cooked, remove it from the pan and set it aside.

In a bowl, whisk together the sour cream, eggs, nutmeg, salt, and pepper. Add the caramelized onions, crispy bacon, and caraway seeds to this mixture and stir everything together.

Preheat your oven to 375°F. Roll out the dough on a floured surface to fit your springform pan. Place the dough in the pan, ensuring it covers the bottom and sides. Pour the onion filling over the dough.

Place the pan in the preheated oven and bake for 40-45 minutes, or until the top is golden brown and the filling is set.