Preheat your oven to 400°F (200°C).
Set medium heat to cook ground beef in large skillet until brown. Cook by adding chopped onion and minced garlic, until the onion turns translucent.
Add the shredded cabbage to the skillet and sauté for a few minutes until it wilts slightly.
Season the mixture with salt and black pepper, stirring well to combine. Remove from heat and set aside.
Add little flour on surface and rol out puff pastry sheet into rectangles.
Cut the rolled-out puff pastry into rectangular shapes, approximately 6x8 inches in size.
Using a spoon, put beef and cabbage filling on one side of each pastry rectangle, making sure to leave some empty space around the edges.
Fold the other half of the pastry over the filling, creating a pocket, and press the edges firmly to seal. For a decorative touch, crimp the edges with a fork.
Place the runzas on a baking sheet lined with parchment paper.
Brush the tops of the runzas with beaten egg to give them a glossy finish.
Bake in the preheated oven for about 20-25 minutes, or until the pastry turns golden brown.
Once cooked, remove the runzas from the oven and allow them to cool slightly before serving.