In a large bowl, whisk together bread flour, whole wheat flour, and salt. Add 1 1/4 cups (300 grams) warm water, Sourdough Starter, and oil; stir together until a shaggy dough forms and no dry spots remain.
Cover and let rise in a warm, draft-free place (75°F/24°C) for 4 hours, folding dough in bowl every 1 hour. (To fold, carefully pull and stretch one side of dough, and fold it over on itself. Repeat for all four sides.) After final fold, cover and refrigerate dough for at least 8 hours or up to 24 hours.
Lightly dust work surface with all-purpose flour. Turn out dough onto surface. Cover and let stand for 20 minutes. Pat dough into a 9x7-inch rectangle, with one short side closest to you. Fold bottom third of dough up; starting at short side where dough is folded, roll up dough into a log. Pull log, seam side down, toward you to create tension on surface.
Lightly spray a 9x5-inch loaf pan with baking spray with flour. Place dough, seam side down, in prepared pan. Cover and let rise in a warm, draft-free place (75°F/24°C) until doubled in size and risen just above sides of pan, 2 1/2 to 3 hours.
Preheat oven to 450°F (230°C).
Lightly dust top of dough with all-purpose flour. Place in oven. Immediately reduce oven temperature to 425°F(220°C).
Bake until golden and an instant-read thermometer inserted in center registers 190°F (88°C), about 30 minutes, covering with foil during final 10 minutes of baking to prevent excess browning. Let cool in pan on a wire rack for 15 minutes. Remove from pan, and let cool completely on a wire rack. Store in an airtight container for up to 1 week.
Notes
Olive oil is added to enrich and soften the loaf to create the bounce and texture of sandwich bread.