Dissolve yeast in 2tbs warm water in a small bowl. Mix together flour, sugar, and 1/2 tsp salt in a large bowl. Add milk, 8 tbs butter and eggs to yeast, mix well then stir into flour. If the flour is too dry and crumbly, add water 1tbs at a time until a dough ball is formed. Turn dough out onto a lightly floured surface and knead until elastic - about 5 minutes. If using a stand mixer, use the dough hook to knead, then turn out and test elasticity by hand kneading. Put dough into an oiled bowl, turn it to coat with oil, cover with a cloth and set aside in a warm place to rise for about 30 minutes. Once the dough has doubled in size, punch down, cover and let rise for additional 30 minutes. While dough is rising, make the filling.
Heat 2tbs oil in a large skillet over medium-low heat. Add onions and cook until soft and starting to brown - about 15 minutes. Increase heat to medium-high, add beef, and brown for 8 minutes. Stir in cabbage, cook for 10 minutes, then add cheese and mustard and season to taste with salt and pepper. Simmer for 5 minutes more, then set aside to cool.
Preheat oven to 350°F. Return dough to a floured surface and divide into 12 equal balls. Roll out into 6" rounds and spoon about 1/3 cup of filling into the center, then fold edges together and pinch closed. Fold tips together and pinch to seal, then place seam side down on an oiled or parchment-lined baking sheet. Let rise for 20 minutes after all bierocks are made, then bake until tops are golden, about 15 - 20 minutes.