In a large mixing bowl, vigorously beat together the fed sourdough starter, 3 cups of flour and 1 1/2 cups lukewarm water. If the dough is not smooth add a little more water, a tablespoon at a time just until a smooth dough forms.
Cover the bowl and leave at room temperature for 4 hours then refrigerate 12 hours or overnight.
Add the remaining 2 cups of flour, sugar and salt to the dough. Either by hand or with a stand mixer fitted with a dough hook, knead the dough until smooth.
Cover the bowl and allow the dough to rise until nearly doubled, smooth and relaxed. This may take anywhere from 2-5 hours. Do not rush this process.
Gently divide the risen dough in half. Gently form two loaves and ideally, place them in a covered stone baker or on a parchment-lined baking sheet. Cover and rise until very puffy, 2-4 hours.
If baking on a parchment-lined baking sheet, place an empty broiler pan in the lower portion of oven. Preheat oven to 425 degrees.
Slash the tops of both loaves and mist with water. Place in preheated oven and if using parchment-lined baking sheet, pour one cup boiling water in broiler pan below loaves and immediately close oven door.
Bake loaves or 25-30 minutes until golden brown. Remove from oven and cool completely on a wire rack.