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1
Stir 1 cup warm water, 1/2 cup all-purpose flour, 1/2 cup whole wheat flour, 1/4 cup rye flour, and yeast together in a large bowl. Cover the bowl with plastic wrap and let sit until the sponge bubbles and doubles in volume, 5 to 6 hours.
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2
Stir 1 cup all-purpose flour, 1 cup whole wheat flour, 1/2 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. Scrape down the the sides of the bowl, cover bowl with plastic wrap, and let dough rise until doubled in volume, 10 hours to overnight.
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3
Line a baking sheet with parchment paper. Dust parchment paper with 1/2 teaspoon all-purpose flour and cornmeal.
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4
Scrape dough out of bowl onto a lightly floured work surface, press down to remove air, and form into a smooth oval loaf. Place dough on the prepared baking sheet. Dust top of loaf lightly with flour, cover with plastic wrap, and let rise until doubled in size, about 1 1/2 hours.
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5
Preheat oven to 450 degrees F (230 degrees C). Place a baking dish filled with water on the bottom rack of the oven.
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6
Remove plastic wrap from risen dough and mist the top of the dough with water.
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7
Bake loaf 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. Transfer bread to a cooling rack and let cool completely before slicing.
