Place water and 465 g bread flour in the bowl of a stand mixer fitted with the dough hook attachment. Mix on low for 2 minutes, or until flour is completely incorporated. Let stand 20-30 minutes.

Meanwhile, mix together starter and the 30 g bread flour. Cover and let stand 20-30 minutes.

Add starter mixture to the mixer bowl along with yeast and salt. Knead on medium-low for 5-7 minutes. Remove from bowl and knead a few times by hand. The dough will be slightly sticky. Dust mixer bowl with flour and place dough in bowl. Cover and let rise until doubled, 90 minutes.

Heat oil in a large skillet over medium heat. Add onions and sprinkle with salt. Cook, stirring occasionally, until onions begin to brown. Remove from heat.

Stir in bread crumbs and poppy seeds and set aside.

Place 4 pieces (18x18 inch) of baking parchment on your counter.

Turn out dough onto a lightly floured work surface and divide into about thirty 30 g pieces, shaping each into a small ball. Place 7-8 on each piece of parchment, flattening slightly and leaving 2-3 inches of space between them. Cover and let rise until very soft, 60-90 minutes.

Preheat oven to 450°F. Place a baking stone in the oven to preheat.

Uncover bialys, use your fingers to make a depression in the center, and scoop about a tablespoon of filling into the depression, pressing down so the onion mixture holds together.

Use a pizza peel or the back of a baking sheet to transfer bialys, parchment and all, to the baking stone.

Bake 15-16 minutes, or until browned. Cool completely.