Combine boiling water, oats and millet in a large bowl. Let cool to 38 degrees C, about 10 minutes.
Dissolve yeast and salt in warm water in a small bowl. Let stand until bubbly, about 5 minutes. Stir into the oat-millet mixture. Add 2 3/4 cups flour; stir until dough forms a loose ball. Cover with plastic wrap; let rise in a warm, draft-free location until doubled in volume, about 2 hours.
Dust a cutting board with 1 tablespoon flour. Turn the dough out onto the board. Tuck in the edges gradually to shape dough into a boule (rustic, French-style ball) without kneading. Cover with a clean dish towel; let rise until nearly doubled in volume, about 40 minutes.
Preheat oven to 245 degrees C. Place a lidded Dutch oven inside.
Make 3 slashes on top of the dough boule using a serrated knife.
Remove hot Dutch oven from the oven carefully. Uncover; sprinkle polenta over the bottom and place boule inside; cover with the lid. Reduce oven heat to 230 degrees C.
Bake in the preheated oven for 28 minutes. Remove the lid and reduce oven temperature to 218 degrees C. Continue baking until top is golden brown, 12 to 14 minutes more. Carefully lift loaf out of pan onto a work surface and gently tap bottom of loaf; if it sounds hollow, bread is done. Let cool for at least 1 hour on a wire rack.
The Dutch oven needs to preheat for at least 20 minutes; do that during the second rise to save time.