Combine the flour and salt in a large mixing bowl. Combine the water, sugar and yeast in a measuring cup, and stir together. Allow the yeast to bloom for 10-15 minutes.
Pour the water/yeast mix into the flour, and stir together into a ragged dough.
Add the sourdough starter. Fold the edges of the ragged dough up and over the starter to enclose it in dough. Using wet hands, pinch the dough 3-4 times to cut it into chunks. Fold the dough back together a few times. Then repeat the pinching and folding for 5-6 minutes, until the dough is fully mixed. See videos above. Cover and rest for 15 minutes.
This dough needs 3-4 more folds. The folds for the rest do not include any pinching and only take about 10 seconds each. Each 15 minutes for the next 45 minutes- 1 hour, uncover the dough, fold the edges up and over the center, all around the outside of the bowl. Then cover and rest again. See videos above.
After the folds are done, cover tightly and allow to rise in a warm place for 1 hour, or until doubled in size.
Once the dough is doubled in size, tip it out onto a floured counter to shape it.
Flour your hands and the top of the dough slightly. Press the dough into a rough rectangle, then fold in the right and left sides in thirds, like a letter. Rotate the dough 90 degrees, flatten it slightly, then fold in the left and right, again, in thirds. Flatten it slightly again. Video above.
Roll the dough up and away from you into a log. Seal it tight by pressing gently on the counter and pulling it back towards you at the same time. See video above.
Pre heat the oven to 375F. Transfer the log to a greased loaf pan, oil the top lightly, and cover loosely with plastic wrap. Allow to rise for 45 minutes-1 hour. It's ready when it doesn't spring back immediately when poked.
Bake for 35-40 minutes until golden brown, and when it sounds hollow when tapped on the underside.
Cool fully on a wire rack before slicing. Once cooled, store in a plastic bag in the fridge for 2-3 weeks. Or in the freezer for up to 2 months.