In a large bowl, combine the water, sourdough starter and 4 cups of flour (2 wheat, 2 white is what I usually do).
Mix well and cover with plastic wrap in a warm place 8-12 hours (I do it over night).
The next day stir in the salt and sugar, then add vital wheat gluten (if using) and flour 1/2 to 1 cup at a time to make a very stiff dough and knead until smooth.
Cover and let rise 2 to 2-1/2 hours.
After 2ish hours, punch down and divide in half.
Knead till smooth and form into rounds.
Place on a greased baking sheet (I put them on a baking sheet covered with finely ground corn meal and then transfer them to my baking stone for baking).
Cover lightly (I use plastic wrap coated with a bit of olive oil so it won't stick) and let rise till double and puffy (1 to 1-2 hrs).
While this is rising mix the 1/2 cup water and 1/2 tsp cornstarch in a small bowl and microwave till boiling, remove from heat and let cool.
Preheat the oven to 400 degrees F and carefully place a small pan of hot water on the bottom rack of the oven.
When done rising, cut two slashes across each other on top of each loaf.
Bake for 10 minutes.
Pull out the rack and baste each loaf well with the cornstarch mixture, then close the oven and allow to bake another 20-25 minutes.
Loaves should be a light golden color and sound hollow when tapped on the bottom. Allow to cool for 2 hrs to develop the sourdough flavor fully if you can wait that long.