Mix 15g bread flour, 15g spelt flour, 30g water, and 30g sourdough starter in a jar.
Set in a warm location to ferment for 5 hours until doubled and bubbly.
Shape the dough into a batard (oval) or boule (round).
Stitch the dough if necessary to strengthen it more.
Place into a banneton or bowl with a towel in the shape of the bread.
Next day: Preheat oven with dutch oven to 500ºF.
Score the dough with a sharp blade.
Bake for 20 mins with lid on, remove lid, lower to 450ºF and bake for another 20 mins.