Day 1: Make the Preferment

1. In a medium bowl, whisk together the flour, yeast and salt. Pour in the water and mix until no bits of dry flour remain. Cover with plastic wrap and let stand at room temperature for 30 minutes. Refrigerate for 8 to 24 hours.

Day 2: Mix the Final Dough

1. Combine the water and flour in a large bowl and mix thoroughly. Let rest for 20 minutes.
2. Add the pâte fermentée, yeast, and salt and mix using wet hands or a Danish dough whisk until everything is completely combined. Do a few fold and turns in the bowl to strengthen the dough.
3. Place in a large bowl, lightly greased, if preferred, cover with plastic wrap, and proof at room temperature until doubled in volume, about 90 minutes. Perform two stretch and folds; one at the 45-min mark and another at the 90-min mark.
4. Cover tightly with plastic wrap and place in the refrigerator for 8 hours or longer depending on your schedule. I let them cold ferment for 8 hours during the day and baked them in the evening of the same day.

Evening Day 2 or Morning Day 3: Shaping the Rounds

1. Transfer dough to a lightly floured surface. Divide into 6 equal pieces (about 80g each) and form each piece into a small bun. Cover with plastic wrap and let rest for 5 minutes.
2. Beginning with the first ball, flatten each one into an approximately 4” disk. Place disks on a parchment-lined baking sheet about 1” apart.
3. Loosely cover with plastic wrap and let proof until they are very soft and hold an indentation when touched, about 60 to 90 minutes.

Prepare the Filling:

1. While the disks are proofing, saute the onions in the olive oil on medium-low heat until they are just turning brown, about 15 minutes or so. They should reduce to about 1/3 their original volume. Place the onions in a bowl and mix with the bread crumbs, poppy seeds and salt. Add more olive oil if the mixture is too dry. Set the mixture aside to cool.

Prepare for Baking:

1. Heat a baking stone or steel in a 475° oven for at least 30 minutes.
2. Uncover the bialys and create a depression in the center of each disk. Place filling in the depression.
3. Slide the parchment onto the baking stone, and bake until golden, about 12-15 minutes. After placing the bialys in the oven, I reduced the heat by 25 degrees to keep the bialys from burning on the bottom before they browned on top.
4. Transfer to a rack to cool for a few minutes. Serve immediately.
5. Leftovers can be stored in an airtight plastic bag for up to 2 days.