1. In a very large mixing bowl, whisk together the active sourdough starter, warm water, granulated sugar, salt, and melted butter until well combined. The mixture should look slightly milky and the sugar and salt should be dissolved.

2. Gradually add the all-purpose flour and instant yeast to the wet ingredients. Use a sturdy spoon or a Danish dough whisk to mix everything until a shaggy, sticky dough forms. There’s no need to knead; just ensure all the flour is incorporated and there are no dry spots.

3. Cover the bowl tightly with plastic wrap or a clean kitchen towel. Let the dough rise at room temperature (ideally 68-72 F) for 8-12 hours, or until it has at least doubled in size and looks bubbly and active. This is often best done overnight.

4. Lightly flour your work surface. Gently turn the risen dough out onto the floured surface. Gently stretch and fold the edges of the dough towards the center to form a rough ball. Then, gently shape it into a rectangular loaf that will fit comfortably into your greased loaf pan (a standard 9×5 inch pan works well). Avoid pressing out too much air. For cheesy sourdough, sprinkle 1/2 cup of shredded cheddar or Parmesan cheese over the dough before forming your loaf. For herbed sourdough, add 1-2 tablespoons of dried herbs like rosemary, thyme, or Italian seasoning to the flour mixture. For honey oat sourdough, replace the granulated sugar with honey and add 1/2 cup of rolled oats to the dough along with the flour. For garlic sourdough, mix in 1-2 teaspoons of garlic powder or finely minced fresh garlic with the dry ingredients.

5. Place the shaped dough seam-side down into a lightly greased 9×5 inch loaf pan. Cover the pan loosely with plastic wrap or a kitchen towel and let it rise in a warm spot for another 2-3 hours, or until it has nearly doubled in size and looks puffy. It should just crest over the edge of the pan.

6. About 30 minutes before baking, preheat your oven to 375 F (190 C). If desired, brush the top of the risen loaf with a little melted butter for an extra golden crust. For optional toppings, sprinkle the top of the loaf with optional: sesame seeds, optional: poppy seeds, or optional: a light dusting of flour for a rustic look.

7. Bake for 35-45 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. If the top is browning too quickly, you can loosely tent it with aluminum foil.

8. Once baked, immediately remove the bread from the loaf pan and place it on a wire rack to cool completely before slicing. This is a crucial step for preventing a gummy texture inside. Cooling can take 1-2 hours.