1 cup "fed" sourdough starter (active and bubbly, any variety is fine)
1 1/2 cups lukewarm water
2 teaspoons instant yeast
1 tablespoon sugar
2 1/2 teaspoons salt
5 cups unbleached all-purpose flour
Combine all of the ingredients, kneading to form a smooth dough.
Allow the dough to rise, in a lightly greased, covered bowl, until it's doubled in size, about 90 minutes.
Gently divide the dough in half; it'll deflate somewhat.
Gently shape the dough into two oval loaves; or, for longer loaves, two 10" to 11" logs. Place the loaves on a lightly greased or parchment-lined baking sheet. Cover and let rise until very puffy, about 1 hour. Towards the end of the rising time, preheat the oven to 425°F.
Spray the loaves with lukewarm water.
Make two fairly deep diagonal slashes in each; a serrated bread knife, wielded firmly, works well here.
Bake the bread for 25 to 30 minutes, until it's a very deep golden brown. Remove it from the oven, and cool on a rack.