Step 1: Make sure you have active sourdough starter ready to bake with. You need 250 grams of active starter.
Step 2: In a large bowl, mix 585 grams of filtered water and 250 grams of active sourdough starter. Lightly mix, and then add 900 grams of all-purpose, unbleached flour. Mix this well and let sit for 30 minutes.
Step 3: Now add 20 grams of salt and 50 grams of filtered water to your dough. I like to premix these in a small bowl and then add to make sure it gets well incorporated. Pinch into your dough, as shown in the photo below, until it is all mixed in.
Step 4: Mix well with your hands, and then you will start doing a series of stretch and folds every 30 minutes for a total of 4 times. This will take 2 hours total.
Step 5: After you have completed 4 rounds of stretch and folds, let your dough sit in a warm spot and long ferment until doubled. This is your bulk fermentation, and will usually take 4-6 hours but will depend on the temperature of your home.
Step 6: After your dough has doubled, empty out the dough onto a lightly floured surface. Split the dough in half, and shape each loaf into traditional sandwich loaf shapes, and thenplace in your loaf pans, seam sides down. Tie a small plastic bags over your pans to hold in moisture. Place in the refrigerator overnight (or for 2 hours minimum).
Step 7: The next day, pre-heat your oven to 375 °F and pull out your loaf pans of dough from the refrigerator. Bake both together for 45 minutes.
Step 8: Once they are done baking, pull them out and slather with unsalted butter. Cool for 5 minutes in the pans, and then flip them out and let them finish cooling on a cooling rack.