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1
Make the filling: Chop the onion in a food processor until it is minced well.
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2
In a small skillet, heat the butter over medium heat.
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3
Add the onion and cook, stirring constantly.
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4
As the onion begins to soften, add the baking soda to speed up the caramelization process.
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5
Cook until the onion is soft and lightly browned.
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6
Stir in the poppy seeds and set the filling aside.
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7
Make the dough: Without washing the food-processor bowl, combine the 2 cups of tapioca starch, the buckwheat flour, and the salt, and pulse until they are fully mixed.
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8
In a medium saucepan, bring the oil and one cup of water to a boil, stirring constantly and vigorously.
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9
Immediately pour the hot mixture into the flour mixture in the food-processor bowl, and process until it is moist with a sand-like texture.
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10
Allow the dough to cool for 20 minutes.
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11
Meanwhile, in a small bowl, combine the yeast, sugar, and 1/3 cup warm water and allow it to proof for 15 minutes.
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12
Add the yeast mixture and egg whites to the dough and pulse to blend it thoroughly.
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13
It will be shaggy, not yet smooth.
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14
Transfer the dough to a medium bowl, cover the dough with plastic wrap, and allow it to rest for 15 minutes.
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15
While the dough is resting, position an oven rack in the center of the oven and preheat to 500 Fahrenheit.
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16
While the oven is preheating, knead up to 1/3 cup of tapioca starch into the dough until the dough is smooth, elastic, and just moist enough to handle without it sticking to your hands.
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17
Divide the dough into 8 equal portions and roll each piece into a smooth ball.
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18
Dust a piece of plastic wrap with tapioca starch and place the balls on the plastic wrap.
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19
Press down on each ball and massage it into a 4- to 5-inch disk as if you were making an individual pizza crust.
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20
Place the disks on a well-greased baking sheet.
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21
Cover the dough and allow it to rise for at least 30 minutes.
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22
After it is done rising, gently create a 2- to 3-inch-wide depression in the center of each disk.
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23
Spoon the onion filling into the depression in each disk and smear it from side to side.
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24
Reduce the oven temperature to 475 Fahrenheit and bake the bialys for about 20 minutes, or until the bottoms are lightly browned.
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25
Remove the bialys from the oven and transfer them to a cooling rack.
