In the bowl of a stand mixer fitted with the paddle attachment, stir together flours, instant sourdough, and salt by hand. Add warm water and oil, and beat at medium-low speed until a sticky dough forms, about 30 seconds. Cover and let rise in a warm, draft-free place (75°F/24°C) for 2 hours; refrigerate for at least 2 hours.
Line a sheet pan with parchment paper, and generously dust with bread flour.
Turn out dough onto a lightly floured surface. To form a boule, gently press dough to an even thickness. Working clockwise, fold in edges of dough toward center, pressing lightly, to form a ball. Turn ball over, seam side down. Using both hands, cup dough, and pull toward you. Turn dough 90 degrees, and repeat until a smooth, tight, sealed boule forms (4 to 5 times). Place dough on prepared pan. Cover and let rise in a warm, draft-free place (75°F/24°C) for 1 hour. (Alternatively, place dough on prepared pan, cover, and refrigerate overnight. The next day, remove dough from refrigerator, and let rise in a warm, draft-free place [75°F/24°C] for 1 hour.)
When dough has 30 minutes left to rise, place a 5- to 7-quart Dutch oven and lid in a cold oven. Preheat oven to 500°F (260°C). (See notes for directions on baking on a sheet pan)
Brush top of loaf with 1 tablespoon (15g) water, and sprinkle with sesame seeds. Using a lame or razor blade, score top of loaf. Carefully remove hot Dutch oven from oven; remove lid, and place bread, still on parchment, in Dutch oven. Cover with lid, and place back in oven.
Immediately reduce oven temperature to 425°F (220°C). Bake for 25 minutes. Remove lid, and bake until an instant-read thermometer inserted in center registers 205°F (96°C), about 15 minutes more. Immediately remove loaf from Dutch oven, and let cool completely on a wire rack.