Combine boiling water, oats, and millet in a large bowl. Let cool to 100 degrees F (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 475 degrees F (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 cornmeal over the bottom and place boule inside; cover with the lid. Reduce oven heat to 450 degrees F (230 degrees C).

Bake in the preheated oven for 28 minutes. Remove the lid and reduce oven temperature to 425 degrees F (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.