In a large bowl, combine the 500g bread flour, 350g warm water, and 100g active sourdough starter. Mix to a shaggy dough, add the 10g salt, and knead by hand for 8 to 10 minutes (or 6 to 8 minutes with a dough hook) until smooth and elastic.

Place the dough in an oiled bowl, cover, and let rise at room temperature until doubled, 4 to 6 hours.

Turn the dough out, divide into 4 equal pieces (about 200g each), shape into tight balls, and let rest 10 minutes. Reshape tightly and place on parchment squares.

Cover and proof at room temperature until puffy and nearly doubled, 2 to 3 hours, or refrigerate overnight for more sour flavor.

Preheat oven to 450°F (230°C) with a cast iron skillet on the bottom rack. Score the tops, slide bowls in, pour about 1 cup boiling water into the skillet, and bake 30 to 35 minutes until deep brown and hollow-sounding.

Cool completely on a rack, cut off the top third, hollow out the interior leaving a ½-inch shell, and fill with soup. Serve immediately.