Activate yeast in warm milk with sugar. Mix with flour, salt, and butter, then knead until smooth. Let rise for 1 hour.
Cook onions in butter over medium heat until golden, about 25 minutes. Cook bacon until crisp.
Whisk sour cream, eggs, nutmeg, salt, and pepper. Stir in onions, bacon, and caraway seeds.
Roll out dough, press into springform pan, and pour filling over it.
Bake at 375°F (190°C) for 40-45 minutes until golden.
