In a small bowl, dissolve yeast in 2 tablespoons warm water. Place flour, sugar, and 1/2 teaspoon salt in a large bowl. Add milk, 8 tablespoons butter, and eggs to yeast, then stir into flour (if dough is too dry, add more water).
Turn dough out onto a lightly floured surface and knead until elastic, 5 minutes. Place in an oiled bowl, cover with a clean dish towel, and set aside to rise until doubled, about 30 minutes. Punch dough down, then set aside to rise for 30 minutes more.
Heat 2 tablespoons oil in a large skillet over medium heat. Add onions and cook until soft, 20 minutes. Increase heat to medium-high, add beef, and brown for 8 minutes. Stir in cabbage, cook for 10 minutes, then add cheese, mustard, salt and pepper. Cook for 5 minutes more, then set aside to cool.
Preheat oven to 350F. Return dough to floured surface and divide into equal balls. Roll out into 6-inch rounds. Spoon about 1/4 cup beef mixture into center of each round, then fold edges in and pinch closed.
Place, seam side down, on an oiled baking sheet and set aside to rise for 20 minutes. Bake until golden 15-20 minutes. Brush with remaining butter and serve.