In a Dutch oven or wide, heavy skillet over medium-high heat, warm the oil until it shimmers. Add the onions and salt, stir to coat with the oil, then lower the heat to medium-low. Cook, stirring regularly, for about 25 minutes, until the onions are soft, entirely wilted, and only slightly tan.

Transfer them to a bowl and stir in the bread crumbs and poppy seeds. Cover and set aside to cool.

Place the bowl of a stand mixer on a kitchen scale and tare the weight to zero. Measure in the flour, water, salt, and yeast. Place the bowl back on the mixer and fit it with the dough hook. On low speed, mix the ingredients together until there are no dry patches of flour showing.

Stop to scrape down the sides of the bowl and increase the speed to medium. Mix until the sides of the bowl are nearly clean, two to three minutes. Cover the bowl with a clean tea towel and let the dough rest for 10 minutes to allow the flour to hydrate evenly.

Uncover the bowl, turn the mixer speed back to medium, and let it run for five full minutes, until the dough is smooth and satiny and the sides of the bowl are clean. The dough will be slightly sticky. Cover the bowl tightly with plastic wrap and let the dough rise for 45 minutes to one hour, until doubled in size.

Place a pizza stone, Baking Steel, or inverted baking sheet on the center rack of the oven and preheat the oven to 450 degrees Fahrenheit (230 degrees Celsius) for at least 30 minutes.

Place a 9 by 13 inch (23 by 33 centimeter) piece of Gefen Parchment on a pizza peel, large cutting board, or an inverted baking sheet. (You need to be able to easily slide the bialy-laden parchment paper from this surface into the oven.) Dust with the cornmeal.

Generously flour a work surface and scrape the dough out of the bowl onto the floury surface. Turn the dough over to coat with flour while deflating, pressing it out into a rectangle. Divide the dough into six equal pieces, each weighing about 85 grams (three ounces). Return to the first portion you weighed and flatten it into a disk, then lift and pull the edges in toward the center to form a ball. Place it seam-side down, and roll the ball under your cupped palm to form a snug, smooth exterior. Repeat with the remaining portions, working in the same order in which they were weighed, to form a total of six balls.

Working sequentially, starting with the first ball, flatten it into a disk five inches (12 centimeters) across. Lift the disk and, holding it by the edge, turn it to form an outer rim, as though working in a pizzeria window, then place it on the parchment paper and use your fingertips to flatten the center. Work diligently to press out any air bubbles, and use your fingertips to dimple the surface in the center of the dough disk, pressing down with vigor, right through the dough until you feel the work surface. The bialys should be flat in the center with a rim. Repeat with the remaining dough balls. By the time you finish, you may notice that the first bialy has shrunk down to about three and a half to four inches (nine to 10 centimeters) across. That’s okay.

Cover the baking sheet with a clean tea towel and proof for 15 to 30 minutes at room temperature, until puffy and a fingerprint remains in the dough when it’s pressed.

Repeat the dimpling process, pressing out a two to three inches (five to seven and a half centimeters) area for the topping and leaving a chubby edge. Place about 10 grams (a scant 1/4 cup) of the onion topping in the center of each disk and spread it until it touches the chubby rim.

Slide the parchment paper with the bialys directly onto the hot surface in the oven and bake until pale tan, 13 to 16 minutes. Transfer on their paper to a wire rack to cool for five minutes, then cover with a clean tea towel so they steam a bit and stay soft.

Bialys will turn stale quickly, so stash them in a bag or covered storage container as soon as they are cool and eat within one day, at most. Better yet, share with another bialy fan because they’re a rare treat. Bialys do not freeze well.