In a large mixing bowl, dissolve yeast in warm water. Let sit 1–2 minutes until slightly foamy. Add sourdough starter, oil, sugar, salt, and 3 cups flour. Beat until smooth. Stir in enough remaining flour to form a soft dough.

Turn dough onto a floured surface and knead gently 20–30 times until smooth. Place in a greased bowl, turning once to grease the top. Cover and let rise in a warm place until doubled, about 1 to 1½ hours.

Preheat oven to 400°F. Punch dough down and divide in half. Roll each portion into a 12×8-inch rectangle. Roll up jelly-roll style, starting from a long side; pinch seams and ends to seal.

Place loaves seam-side down on greased baking sheets. Tuck ends under, cover, and let rise until doubled, about 30 minutes.

Using a sharp knife, make 4 shallow diagonal slashes across the top of each loaf.

In a small saucepan, whisk together water and cornstarch. Cook over medium heat, stirring constantly, until thickened.

Brush some of the cornstarch wash over loaves. Bake for 15 minutes. Remove from oven, brush with remaining wash, then return to oven and bake 10 minutes longer or until lightly golden brown.

Remove loaves from pans and cool on wire racks before slicing.