Remove 60 grams of sourdough starter from refrigerator. Feed with 30 grams of filtered water and 30 grams of bread flour. Leave for 3 hours to come to room temperature.
Measure 60 grams of fed sourdough starter and either discard the rest or add it to the starter in the refrigerator. Add the filtered water, Kosher salt, and bread flour to the starter in a bowl. Stir to combine with dough whisk or wooden spoon. Dough will be shaggy.
Cover and let rest for 3 hours. With wet hands, fold dough over on itself several times. Cover with foil and allow dough to ferment for 3 additional hours.
Generously dust 7-inch banneton with rice flour (See Note 3).
Transfer dough to a lightly floured work surface. Shape into a ball with a smooth, unbroken surface. Transfer smooth-side down to the banneton. Pinch together the top edges of the dough together. Cover banneton and refrigerate overnight.
The next day, remove the banneton from the refrigerator. Let rise until the dough springs back slowly and retains a slight indentation when poked gently with a wet finger, 4 to 5 hours.
Preheat oven to 400° F.
Place parchment lined baking sheet on top of banneton. Gently flip. Carefully remove banneton. Make two slits in the center, like a cross. Mist entire surface lightly with water.
Bake in preheated oven until golden brown, 25 to 30 minutes. If desired, brush with egg wash after 15 minutes; return to oven for remaining time.
Let cool completely on a wire rack.
Yield: 1 7-inch round sourdough loaf.