Dissolve the yeast in warm milk in a small basin and let aside for 5 minutes, or until foamy. Combine the sugar, softened butter, salt, egg, and yeast mixture in a large mixing basin.
Mix in the flour gradually until a soft dough forms. Knead the dough until smooth and elastic on a floured surface. Place the dough in an oiled basin, cover, and let aside for approximately 1 hour, or until doubled in size.
Cook the ground beef in a large pan over medium heat until browned. Cook until the chopped onions and shredded cabbage are softened in the skillet.
Season the filling to taste with salt and pepper. Any extra liquid should be drained from the skillet.
Prepare the oven to 350 degrees Fahrenheit (175 degrees Celsius). Punch the rising dough down, divide it into two equal pieces, and roll out one half to fit the bottom of a prepared baking dish.
Evenly distribute the meat, onion, and cabbage mixture throughout the rolled-out dough. Sprinkle shredded cheese over the filling if desired. Roll out the second amount of dough and close the edges to cover the filling. Cut a few slits on the top to let steam escape during baking.
Bake the Runza Casserole for 25-30 minutes, or until the top is golden brown, in a preheated oven. Allow for a few minutes of cooling before slicing the dish. Serve the Runza Casserole in squares while still warm. Serve mustard on the side as a typical complement.
Tips and Variations:
Experiment with several cheeses or combine them for a gooey, savory twist.
For a vegetarian option, replace the ground beef with plant-based alternatives.
Make the filling and dough ahead of time for speedy assembly on busy days.
Unbaked Runza Casserole may be frozen for later use. When ready to eat, thaw and bake.