1. In a small bowl, whisk together water, sugar, and yeast; let stand 10 minutes.

2. Add yeast mixture to the bowl of an electric mixer fitted with the dough hook attachment, along with both flours, oat bran, flaxseed, walnuts, sea salt, olive oil, and rosemary; mix until a dough forms, about 10 minutes.

3. Coat a large bowl with oil. Add dough to bowl and turn to coat. Cover bowl with plastic wrap; let stand in a warm place until dough has doubled in size, 2 to 3 hours.

4. Line a baking sheet with parchment paper and sprinkle with cornmeal. Punch down dough and shape into a round loaf. Place dough on prepared baking sheet. Cover with a clean towel; let stand in a warm place for 30 minutes.

5. Preheat oven to 500 degrees with a rack set in the center of the oven. Transfer baking sheet to oven and bake until browned, about 25 minutes. Let bread cool on a wire rack before serving.