In a large mixing bowl, combine your active sourdough starter and warm water. Use a wooden spoon to create a smooth mixture.
Gradually add the bread flour and salt to the wet ingredients, stirring continuously until a shaggy dough forms.
Once your dough starts coming together, transfer it to a floured surface. Knead it by hand for about 10 minutes until you achieve a smooth dough. Add more flour as needed to prevent sticking.
Place the dough in a greased bowl and cover it with plastic wrap or a damp cloth. Let it rest in a warm place for the first rise, usually about 4-6 hours or until it has doubled in size.
Once the dough has risen, gently punch it down to release excess air. Divide it into equal portions, usually 4-6 pieces depending on the size you desire.
Shape each portion into a round ball and place them on a baking tray lined with parchment paper, seam side down.
Cover the dough balls with a clean tea towel and let them rest for an additional 30 minutes
Preheat your oven to a hot 450°F (230°C) and place a baking stone on the middle rack. If you don't have a baking stone, a preheated cast-iron Dutch oven will do the trick.
Take each dough ball and, with a sharp knife or a scoring lame, make a deep cross-like incision on top of the bread.
Dust the top of the dough with rice flour for that classic rustic touch. Using a pastry brush, lightly brush each dough ball with water to create a golden-brown crust when baking.
Carefully transfer the dough, parchment paper and all, onto your preheated baking stone or into your preheated Dutch oven. If using a Dutch oven, cover it with the lid.
Bake for about 20 minutes. After this time, remove the lid (if using a Dutch oven) and continue baking for an additional 15-20 minutes or until the bread bowls are golden brown and sound hollow when tapped on the bottom.
Remove the bread bowls from the oven and place them on a wire rack to cool.
Once they've cooled, carefully slice off the top of each bread bowl using a serrated knife. Hollow out the center to create a bowl shape, being sure to leave a sturdy edge for holding your delicious soup.