Is It a Bierock or a Runza? These Savory Midwestern Buns Are Delicious Either Way

Name: Is It a Bierock or a Runza? These Savory Midwestern Buns Are Delicious Either Way
Ingredients: 1/2 cup (120ml) whole milk, room temperature; 1/2 cup (120ml) warm water, plus more as needed (see notes); 52g granulated sugar (about 1 3/4 ounces; 1/4 cup); 28g unsalted butter (about 1 ounce; 2 tablespoons), melted and cooled slightly; 1 large egg, room temperature; 496g all-purpose flour (17 1/2 ounces; about 3 1/2 cups), plus more for shaping; 7g (2 1/4 teaspoons) instant yeast; 2 teaspoons Diamond Crystal kosher salt; for table salt, use half as much by volume or same quantity by weight; Nonstick spray; 1 tablespoon (15ml) neutral oil, like canola or vegetable; 1 pound ground beef (16 ounces; 454g), 85% lean; 1 large yellow onion, diced (10 ounces; about 2 cups); 2 teaspoons Diamond Crystal kosher salt; for table salt, start with half as much by volume, plus more to taste; 1 teaspoon freshly ground black pepper; 1 teaspoon paprika; 1/2 teaspoon garlic powder; 1/2 teaspoon onion powder; 1/2 medium green cabbage (20 ounces; 567g), cored, quartered, and thinly sliced (about 6 cups); 1/2 cup (120ml) water or low-sodium beef broth; 1 large egg; 1 teaspoon water; Mustard or other condiments of choice, for serving (optional)