Make soaker: Put polenta and bulgur in a small bowl, add 1/2 cup boiling water, and let stand until grains are softened, 3 to 4 hours.
Make dough: Combine 3 1/2 cups flour, the yeast, and salt in a bowl of a stand mixer or large mixing bowl. Add 1 cup plus 2 tbsp. room-temperature water, the soaker, and 2 tbsp. oil, mixing on low speed with a dough hook or stirring by hand with a wooden spoon until well incorporated. The dough should be soft and tacky; adjust with flour or water as necessary. If using a mixer: Beat on medium speed, scraping down inside of bowl occasionally, until dough is smooth, soft, and stretchy (it should still feel tacky), 12 to 15 minutes. If kneading by hand: Transfer to a lightly floured work surface and knead with damp hands, adding only a light coating of flour and wetting hands as required to prevent sticking, until dough is smooth, soft, and stretchy (it should still feel tacky), 12 to 15 minutes.
Let dough rest 5 minutes for the moisture to even out. Make any final flour or water adjustments, using mixer or by hand, adding water or flour, until dough feels soft, supple, and tacky but not sticky.
Transfer dough to a 2-qt. glass measure, if you have one, or put in a medium bowl. Cover bowl and let dough rise at room temperature until it's 1 1/2 times its original size, 45 minutes to 1 1/2 hours; when touched gently, it should spring back slowly.
Punch dough down, knead a couple of times, and shape into a smooth 9-in. oval. Set in an oiled 5- by 9-in. loaf pan. Brush top with oil and cover with plastic wrap. Let rise until dough comes to top of pan and holds a small dent when pressed with a finger, 30 minutes to 1 1/2 hours.
Meanwhile, preheat oven to 375°. When dough has risen, brush with butter and bake until bread is nicely browned and an instant-read thermometer inserted into center registers at least 195°, about 40 minutes.
Let bread cool in pan on a rack about 10 minutes, then loosen from pan (if it sticks, cool in pan another 5 minutes). Cool at least 1 hour on rack.
Make ahead: For soaker, up to 1 day, chilled. Bring to room temperature before using.
*Find whole-grain polenta and corn grits such as Community Grains and Bob's Red Mill at well-stocked grocery stores, communitygrains.com, and bobsredmill.com