Add starter to a bowl with warm water and 1 1/2 cups flour. Stir to combine, then let rest loosely covered on a counter for 2-3 hours.
1/2 cup active sourdough starter, 3/4 cup warm water, 1 1/2 cups flour
Refrigerate overnight.
To make the dough
Remove sponge from fridge and let sit on counter for one hour.
Scrape sponge into a large mixing bowl. Add sugar, salt, and yeast (if you choose to use it) along with the remaining 1 1/4 cups flour. Mix together, then knead 5-7 minutes once dough forms.
If you do not have a Dutch oven, gently mist the boules with water, and slide them onto a baking sheet lined with cornmeal. They may take a little less time to bake when exposed directly to the oven's heat.
For more tips and tricks, be sure to read the full article above.