Combine bread flour, 1 cup whole wheat flour, flax seeds, salt, and yeast in a large bowl. Add cool water; mix, without kneading, until dough is wet and sticky. Cover bowl with plastic wrap; let sit at room temperature until dough is bubbly and swelled, about 12 hours.
Grease a loaf pan with oil; dust with 1 1/2 teaspoons whole wheat flour. Place dough in pan; dust the top with remaining whole wheat flour. Poke holes in the dough using a chopstick or make cuts with a knife.
Cover loaf pan with plastic wrap. Let dough rise until doubled in volume, about 1 hour.
Preheat oven to 450 degrees F (230 degrees C). Place an oven-safe cup of water in the oven.
Bake in the preheated oven until chestnut-brown on top, about 45 minutes. Lift bread out of the pan using pot holders; let cool thoroughly on a rack.