A bialy’s centre is a little landing place for onion. Stretch it right through and you have lost the feature that holds the filling; leave it too thick and the middle can rise into a mound.
For a first batch, use a tested dough and schedule such as King Arthur Baking’s classic bialys. Its shaping and filling instructions are more useful than borrowing the baking time from a different bread roll.
Make room without making a hole
Work with one rested piece at a time. Preserve a fuller outer rim while gently thinning the centre into a broad depression. If the dough springs back immediately, follow the recipe’s resting guidance instead of repeatedly pulling harder. Inspect the underside before filling: it should still form a continuous base.
Shape the whole tray with roughly comparable centres. A very thick middle and a paper-thin middle will behave differently even when the pieces weigh the same.
Onion is a filling, not a mountain
The King Arthur recipe cautions against an overly generous quantity of onion because its moisture affects the centre’s bake. Chop and prepare the onion as directed, then distribute a modest amount in the depression. Keep the raised rim visible.
A useful first-batch note has three parts: how deeply you shaped the centre, approximately how much filling you used, and what the cooled bread looked like when cut. Photograph one cross-section. This gives you something more concrete to adjust than “make it more like the bakery.”
Keep the recipe’s oven settings unchanged for that first attempt. If the centre rises too much, examine shaping and filling before changing temperature, proofing and flour all at once. Several simultaneous changes make it difficult to learn which one helped.
Serve while fresh for the best contrast between the tender middle and outer crust. If storing some, let them cool before packaging and use the recipe’s storage instructions.