In a small bowl or measuring cup, add the yeast and sugar to the lukewarm water. Allow to rest for 5-10 minutes, or until yeast is foamy/bubbly.
In a large mixing bowl, whisk together the flour and salt. Add the sourdough starter and the yeast mixture, and stir until combined. Dough will be very sticky!
Cover the bowl with a tea towel and set in a warm place to rise for an hour.
Preheat the oven to 425 degrees. Grease two oven-safe bowls (see note) with about a tablespoon of butter each.
Gently stir down the risen dough. Scrape half of the dough into each bowl. Cover the bowls again and allow to rise for 20-30 minutes, or until the dough reaches just below the rim (for a larger bowl) or just above it (for a smaller one).
Bake for 15 minutes. Reduce heat to 375 degrees and continue baking for an additional 15-17 minutes or until loaves are golden brown.
Remove loaves from oven and turn out onto a cooling rack. Let cool for 10 minutes before slicing.
Notes
Pyrex bowls are the perfect baking vessels for this bread, preferably 1 to 1.5 qt size. The ideal size is a Pyrex 322 bowl. Do not use a bowl larger than 2 qts. If you do not have bowls this size, the original post says that you can bake 3/4 of the dough in a loaf pan and bake the remaining dough in muffin tins or popover pans.Makes 2 loaves.