To prepare the dough:

1. In the bowl of a stand mixer fitted with a dough hook attachment, add the warm water and yeast. Stir to combine, and let stand 10 minutes.
2. Mix in the sugar, butter, egg, salt and 3 1/2 cups of the flour. Beat on low until smooth. Working in 1/2 cup portions, add the remaining flour until the dough pulls together.
3. Transfer the dough to a lightly oiled bowl. Cover and refrigerate for 2 hours up to overnight.

To prepare the filling:

1. In a large heavy skillet over medium heat, brown the meat. Add the onion, cabbage, salt and pepper, and simmer for 30 minutes. Remove from heat, and allow to cool until the mixture is lukewarm.
2. Coat a baking sheet with nonstick cooking spray. Set aside.
3. Transfer the dough to a flat working surface. Punch down the dough, then divide the dough into 20 equal pieces. Flatten each portion of dough, and fill with 2 tablespoons of the filling. Fold the dough over, and seal the edges. Transfer each filled bierock to the prepared cookie sheet. Allow to rise for 1 hour.
4. Preheat oven to 350 degrees F.
5. Bake the bierocks until golden brown, about 25 minutes. Brush with the melted butter, and serve immediately.