1. Activate Your Starter and Mix Wet Ingredients: In a large mixing bowl, combine your active sourdough starter with the warm water, granulated sugar, and salt. Stir well until the sugar and salt are dissolved and the starter is mostly incorporated. Add the olive oil or melted butter and mix again. The mixture should look milky and slightly bubbly from the starter.

2. Combine Dry Ingredients and Form a Shaggy Dough: In a separate bowl, whisk together 5 cups of all-purpose flour and the vital wheat gluten (if using). Gradually add the dry ingredients to the wet ingredients, mixing with a wooden spoon or your hands until a shaggy, cohesive dough forms. It will be sticky at this stage.

3. Knead the Dough for Smoothness: Turn the dough out onto a lightly floured surface. Knead for 8-10 minutes until the dough becomes smooth, elastic, and less sticky. If the dough is too sticky to handle, add flour one tablespoon at a time, being careful not to add too much. You’re looking for a dough that springs back when gently poked.

4. First Rise (Bulk Fermentation): Lightly oil a clean large bowl. Place the kneaded dough in the bowl, turning it once to coat all sides with oil. Cover the bowl tightly with plastic wrap or a damp kitchen towel. Let it rise in a warm spot for 4-6 hours, or until doubled in size. Alternatively, for a deeper flavor, you can refrigerate it overnight for 8-12 hours.

5. Shape the Loaf: Gently punch down the risen dough to release the air. Turn it out onto a lightly floured surface. Shape the dough into a rectangular loaf that will fit comfortably into your prepared loaf pan (a standard 9×5-inch pan works well). To do this, flatten the dough into a rectangle, then fold the top third down to the center, and the bottom third up over the top. Pinch the seam closed, then roll the dough gently to create a taut surface.

6. Second Rise (Proofing): Lightly grease your loaf pan. Place the shaped dough seam-side down into the prepared pan. Cover loosely with plastic wrap or a light kitchen towel. Let it rise in a warm place for 1-2 hours, or until it has nearly doubled in size and looks puffy. It should just barely peek over the top of the pan.

7. Preheat Oven and Bake: While the dough is proofing for its second rise, preheat your oven to 375 F (190 C). Once the dough is fully proofed, place the loaf pan in the preheated oven. Bake for 35-40 minutes, or until the crust is golden brown and the internal temperature reaches 200-210 F (93-99 C) when checked with an instant-read thermometer.

8. Cool Completely Before Slicing: Once baked, remove the bread from the oven and immediately transfer it from the loaf pan to a wire cooling rack. Allow the bread to cool completely for at least 1-2 hours before slicing. Slicing too early can result in a gummy texture.