In a large bowl whisk together bread flour, instant yeast, sugar, and salt. Add warm water (about 105–115°F / 40–46°C) and vegetable oil. Stir with a wooden spoon or dough hook until a shaggy mass forms. Let rest 10 minutes (autolyse) to hydrate the flour.

Knead

Knead the dough by hand on a lightly floured surface for 8–10 minutes until smooth and elastic, or use a stand mixer with the dough hook for 6–7 minutes. The dough should be slightly tacky but pull into a smooth ball.

First rise

Place dough in a lightly oiled bowl, cover with plastic wrap or a damp towel, and let rise in a warm draft-free place until doubled in size, about 60–90 minutes depending on room temperature.

Onion topping

Prepare onions

While dough is rising, mince or finely dice the yellow onion to yield about 1 1/2 cups. Heat a skillet over medium heat and add the butter and olive oil.

Cook onions

Add the diced onion to the skillet and season with 1 tsp salt, 1/2 tsp black pepper, and 1/2 tsp dried thyme if using. Cook, stirring occasionally, until the onions are softened and just beginning to brown, about 8–12 minutes. Remove from heat and let cool to room temperature. The topping should be moist but not oily.

Finish & assembly

Divide dough

After the dough has doubled, gently deflate it and turn it out onto a lightly floured surface. Divide the dough into 8 equal pieces (about 100–110 g each). Shape each piece into a smooth round ball and let rest covered for 10 minutes.

Shape bialys

Working with one ball at a time, flatten each ball into a 4–5 inch (10–13 cm) disk with your fingers, leaving a slightly thicker edge. Using your thumb or the back of a spoon, press a shallow well in the center of each disk about 1/2 inch deep, creating the characteristic cavity for the onion filling.

Fill

Spoon about 2 tablespoons of the cooled onion topping into each cavity, spreading slightly but leaving the edges free. If the topping seems wet, drain briefly on paper towel before filling so it doesn't make the dough soggy.

Second rise

Place shaped bialys on a baking sheet dusted with cornmeal or semolina, spacing them 1–2 inches apart. Cover loosely with plastic wrap or a damp towel and let rise at room temperature for 30–40 minutes until slightly puffed.

Preheat oven and egg wash

Preheat the oven to 450°F (230°C) with a rack in the middle. Beat the egg with 1 tsp cold water to make an egg wash. Gently brush the outer rim of each bialy (avoid brushing the onion filling) with the egg wash and sprinkle poppy seeds on the rims if using.

Bake

Bake the bialys in the preheated oven for 12–15 minutes until the crust is golden brown and the filling is set. Rotate the pan halfway through baking for even color.

Cool

Remove from oven and transfer to a wire rack. Cool for at least 10 minutes before serving. Bialys are best the same day; to reheat, warm in a 350°F (175°C) oven for 5–7 minutes.