First, make the dough. Combine 1/2 cup warm water, sugar and yeast. Let sit until foamy. Then add remaining water, salt and flours. Knead for 8 minutes by hand if you’e tough or with a dough hook on an electric mixer until dough is soft and not sticky and roll into a ball. Add a tiny bit of flour if your dough is sticky or water if it seems dry.
Place dough in an oiled bowl and let rise for 1 1/2 hours or until tripled in size. Punch down and let rise another 45 minutes until doubled in size. On a floured surface, roll dough into a log and cut into eight pieces.
Preheat oven to 425 degrees F. Line two cookie sheets with parchment paper, and place pieces on the cookie sheets flattening slightly into rounds. Cover with plastic wrap and let rise another 30 minutes. Meanwhile, make filling by combining onion, poppy seeds, olive oil, garlic, salt and pepper. With your fingers make a 1-inch indentation in each bialy and divide filling amongst bialys.
Bake for 6-7 minutes and then reverse bialys around in the oven from front to back and bake another 5-6 minutes until lightly browned, careful not to overcook.
Freeze any extras wrapped in foil and placed in a plastic bag, and toast those before noshing.