In a large mixing bowl, mix milk and sugar to dissolve the sugar. Sprinkle yeast over the milk mixture and let stand for 5-10 minutes, until the yeast softens and starts to foam.
Whisk the mixture to combine and whisk in 4.5 cups of flour.
Add melted butter, egg, and salt. Whisk to incorporate.
Stir in the remaining flour ¼ cup at a time, using only as much as you need to get the dough to come together. Turn the dough onto your counter and knead, for 10-15 min, until a soft, smooth dough forms, adding flour as needed. (Your finished dough should be tacky, but not stick to your hand or your kneading surface.)
Shape dough into a round; place it in a greased bowl, turning to coat the dough. Cover the dough loosely with plastic wrap or a damp towel and place in a warm, draft-free place to rise until doubled in bulk, about 1 hour.
While the dough rises, make your filling. In a large frying pan, cook potatoes until the are 80% cooked. Add beef and brown meat over medium-high until mostly cooked, 5-7 min.
Drain as much of the grease from the pan as you can, while not losing the meat from the pan. Return the pan to the heat and add onions and garlic. Cook 2-3 min, until they begin to soften.
Add cabbage and cook 7-10 minutes, until cabbage is tender.
Remove from heat and set aside to stuff bierocks.
Combining & Cooking
Knock down the risen dough ball and turn it onto your work surface.
Divide dough into 16 balls (roughly 3 oz each). Flatten each ball to a circle appx 5” in diameter. (If the dough springs back, flatten as much as you can, cover, and let the dough rest for 3-5 min before attempting to flatten further.)
Spoon 2-3 large tablespoons of filling onto the center of each circle, leaving the edges clean.
Wet edges of dough and pull together and pinch them to seal the dough completely. Continue until all the dough and filling has been used.
Place the shaped bierocks on a greased baking sheet and let rise, covered 30-45 min, until roughly 1.5x their original size.
During the last 10 minutes of rising time preheat your oven to 375F.
Brush the bierocks lightly with milk, egg wash or butter and bake for 20-25 min, until golden brown and hollow sounding when tapped.