Prepare the Dough: Warm the milk in a saucepan until it's just starting to bubble (scald it). Stir in 3 tablespoons butter until melted, then cool to lukewarm. In a large bowl or stand mixer, combine 2 cups flour, yeast, sugar, and salt. Add the milk mixture and beaten egg. Mix until smooth, then gradually add remaining flour. Knead for 5-7 minutes until soft and elastic. Cover and let rise in a warm spot for 1 hour or until doubled.

Make the Filling: In a large skillet over medium heat, cook diced bacon (if using) until crisp. Add ground beef and cook until browned. Stir in chopped onion and cook until soft, about 5 minutes. Add shredded cabbage, mustard, salt, and pepper. Cook until cabbage is tender, 7-10 minutes. Let cool. Stir in cheese if desired.

Assemble the Bierocks: Punch down the risen dough and divide into 12 equal pieces. Roll each into a 6-inch circle. Place about 1/3 cup filling in the center. Fold edges over and pinch to seal. Place seam-side down on a parchment-lined baking sheet. Cover and let rise for 30 minutes.

Bake: Preheat oven to 350°F (175°C). Bake for 25-30 minutes until golden. Brush with melted butter for shine.