Mix the Dough: In a large bowl, combine the bread flour, salt, and unrefined sugar. Add the sourdough starter. Gradually add the warm water and olive oil, mixing until a sticky dough forms and all the flour is well incorporated. You can also use a stand mixer with a dough hook attachment. Cover with a damp kitchen towel.
4 1/8 cups bread flour, 1 cup + 1tbsp sourdough starter (recently fed active starter), 1 1/4 cups warm water (filtered), 3 tbsp olive oil, 1 tsp granulated sugar, 2 tsp salt (or 1 3/4 tsp for lower salt)
Rest: Let the dough rest for 30 minutes.
Stretch-and-folds: Perform 2 sets of 4 stretches and folds with your wet hands. The dough should look smooth and elastic. If the dough breaks easily after and is very sticky still, let it rest for another 30 minutes and perform another set of 4 stretch and folds. Cover with a damp towel and let it ferment at room temperature.
Bulk Ferment: Fermentation times can vary from one kitchen to another. For me it took 4 hours (bulk fermentation starts when you first mix the dough, not after first stretch and folds) to double in size (in the summer it would take less for me). Please be aware that it can take less or longer for you.
Your dough is ready when it has almost doubled in size, it has a domed appearance, is jiggly, light and airy (lots of small bubbles on the bottom and larger bubbles on top).
Shape the dough: Flour your work surface and put the dough on. Flour your hands and gently stretch the dough into a rectangle by grabbing the sides from underneath.
Proceed by folding the dough onto itself from all sides. Make sure you gently press to seal the sides. Once you have a square shape fold one more time in half and gently pinch the sides in the middle and do some stitching with your fingers to properly seal the dough.
Prepare your loaf pan with parchment paper. Use a bench scraper to pick up the shaped loaf and turn it side up in the loaf pan.
Wet your hands and gently run them on the surface of the dough without pressing. This helps smoothen the crust of the loaf.
Proof: Let the dough proof in the loaf pan for about 30 minutes. Meanwhile preheat your oven to 480°F (250°C). Mine takes 30 minutes to preheat. The dough is ready to bake if when you gently push one finger into the dough it springs back slowly and not fully. The dough needs more time to proof if the dough springs back quickly and fully. If the dough does not spring back at all your dough is over proofed and likely collapsed. Bake it anyway and try again another time as "practice makes perfect". The bread will still be edible.
Bake: Put the bread in the oven and add a cup of hot water in an oven safe dish to create steam (this will help with oven spring). Close the door immediately and do not open until you need to check if the bread is done. Bake the sourdough sandwich loaf for 30-35 minutes at 480°F (250°C) or until golden brown on top.
Cool and Enjoy: Let the bread cool in the pan for 10 minutes before transferring it to a wire rack to cool completely. Slice and enjoy!