1. In a large mixing bowl, whisk together the active sourdough starter, warm milk, honey, egg, and melted butter until well combined. The mixture should look smooth and slightly frothy from the starter.

2. In a separate medium bowl, whisk together the all-purpose flour, oat bran, granulated sugar, baking powder, baking soda, and salt. Ensure there are no lumps and all ingredients are evenly distributed.

3. Gradually add the dry ingredients to the wet ingredients, mixing with a wooden spoon or spatula until just combined. Be careful not to overmix; a few streaks of flour are fine. The dough will be sticky.

4. Cover the bowl with plastic wrap or a clean kitchen towel and let it rise in a warm place for 2-3 hours, or until it has visibly increased in size and looks bubbly. Alternatively, for a deeper sourdough flavor, you can refrigerate it overnight (8-12 hours) and let it come to room temperature for about 30 minutes before proceeding.

5. Preheat your oven to 375 F (190 C). Lightly grease and flour a 9×5-inch loaf pan, or line it with parchment paper, leaving an overhang on the long sides to easily lift the loaf out later.

6. Gently scrape the dough into the prepared loaf pan, spreading it evenly with a spatula. You can optionally sprinkle a little extra oat bran on top for a rustic look.

7. Bake for 35-40 minutes, or until the top is golden brown and a wooden skewer or toothpick inserted into the center comes out clean. If the top starts to brown too quickly, you can loosely tent it with aluminum foil.

8. Once baked, remove the loaf from the oven and let it cool in the pan for about 10-15 minutes before carefully transferring it to a wire rack to cool completely. This prevents the bottom from becoming soggy.