Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.

Melt the butter in a skillet over medium heat. Add the diced onion and cook until translucent, about 5 minutes.

Add the finely chopped mushrooms to the pan. Cook, stirring occasionally, until all the liquid released by the mushrooms evaporates and they begin to brown, approximately 8-10 minutes.

Stir in the fresh thyme, sour cream, salt, and pepper. Cook for 1 more minute, then remove from heat and allow the filling to cool completely.

Roll out the thawed puff pastry sheet slightly on a lightly floured surface and cut it lengthwise into three equal long strips.

Spoon a line of the cooled mushroom filling down the center of each pastry strip.

Fold one side of the pastry over the filling to meet the other side, creating a long log. Pinch the edges tightly to seal.

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

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

Bake for 20-25 minutes until the pastry is puffed and deep golden brown. Serve warm.