Stir warm water, flours (all-purpose flour, whole wheat, and rye), and yeast together in a large bowl until well combined. Cover the bowl with plastic wrap and let sit until bubbly and doubled in volume, 5 to 6 hours.

Stir all-purpose flour, whole wheat flour, water, sunflower seeds, polenta, flax seeds, salt, and honey into sponge with a wooden spoon until a very sticky dough ball forms, about 3 minutes. Cover and let rise until doubled in volume, 10 hours to overnight.

Line a baking sheet with parchment paper. Dust parchment paper with 1/2 teaspoon all-purpose flour and cornmeal.

Scrape dough out of the bowl onto a lightly floured work surface; press down to remove air, then form into a smooth oval loaf. Place dough onto the prepared baking sheet. Dust top of loaf lightly with remaining 1/2 teaspoon all-purpose flour, cover with plastic wrap, and let rise until doubled in volume, about 1 1/2 hours.

Preheat the oven to 450 degrees F (230 degrees C). Place a baking dish filled with water on the bottom rack of the oven.

Remove plastic wrap from risen dough and mist the top of the dough with water.

Bake in the preheated oven, misting the top of the loaf with water every 8 to 10 minutes, until loaf is golden and sounds hollow when tapped, 30 to 35 minutes total.