To make the dough: Combine the flour, salt, yeast, and onion powder in the bowl of a stand mixer fitted with a beater attachment. Switch to a dough hook, add the water and mix until just combined, 1 or 2 minutes at low speed. Stop the mixer, cover the bowl, and let sit for 20 minutes., After this rest, uncover the bowl and continue kneading at medium speed for 8 minutes, or until the dough is smooth and elastic., Cover and refrigerate the dough (in an oiled bowl if desired) overnight for a slow, cool rise that will develop the dough's flavor., Next day, divide the dough into 12 pieces (they'll weigh just under 4 ounces each) and round each into a ball. Place on a lightly oiled baking sheet, cover with greased plastic, and let rise at room temperature for 1 hour, or until puffy., Preheat the oven to 475°F., To make the filling: Pulse the onion in a food processor until very finely chopped, but not liquid. Mix in the poppy seed, salt, and pepper. Heat the oil in a saucepan and cook the onion mixture for a few minutes over medium-low heat until the liquid cooks off and the onion is very slightly caramelized. Remove from the heat and cool., To shape and bake: Lightly sprinkle 2 baking sheets with farina or cornmeal. Take each dough ball and stretch it into a bagel shape about 6" to 7" in diameter, without puncturing the center, and leaving a wide, flat indentation where the hole would be. Place a few inches apart on the prepared baking sheets, 6 per pan. Place a scant teaspoon of the onion filling in the indentation and spread it out with your fingers. Don't overfill; a little goes a long way., Bake for 8 to 10 minutes, until light golden brown. Remove the bialys from the oven and serve warm; or cool on a rack and save for toasting later.