In a stand mixer bowl with a dough hook, combine flour, yeast, sugar, and salt. Add warm milk, melted butter, and sourdough discard. Mix on low until a shaggy dough forms, then knead on medium for 5-7 minutes until smooth and elastic.

Place the dough in a lightly oiled bowl, cover, and let rise in a warm place for 60-90 minutes, or until doubled in size.

While dough rises, melt the butter for the filling. Stir in minced garlic, parsley, and salt. Set aside.

Punch down the dough and roll it into a 12x20 inch rectangle. Brush with the garlic butter and sprinkle with mozzarella cheese. Cut into five 4-inch wide strips. Stack the strips, then cut the stack into six equal smaller stacks.

Arrange the stacks in a greased 9x5 inch loaf pan, cut side up. Cover and let rise for 30-45 minutes, until puffy.

Preheat oven to 375°F (190°C). Bake for 30-35 minutes until golden brown. Let cool for 10 minutes before serving.