In a small bowl, dissolve the sugar and yeast in the warm milk and let it sit for 5 minutes until frothy.

In a large mixing bowl, combine the flour and a pinch of salt. Pour in the yeast mixture, melted butter, and egg yolks. Knead until a smooth, elastic dough forms, then cover and let rise in a warm place for about 1 hour.

While the dough rises, prepare the filling. Sauté the diced onion in a pan with a little oil until translucent.

Add the chopped mushrooms to the pan and cook until their moisture evaporates.

Stir in the drained, chopped sauerkraut and cook for another 10 minutes on medium heat. Season generously with salt and pepper, then let the filling cool completely.

Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper.

Punch down the risen dough and divide it into two parts. Roll each part out into a long rectangle on a floured surface.

Spread the cooled filling along the long edge of each rectangle, then roll the dough up tightly like a log to enclose the filling.

Cut the logs into 2-inch wide pieces and place them seam-side down on the baking sheet.

Brush the tops with the beaten whole egg and sprinkle with caraway seeds if desired.

Bake for 20-25 minutes until the pastries are deep golden brown. Serve warm.