Make the dough: In the bowl of a stand mixer fitted with a hook, stir together syrup, yeast, and 1 ½ cups water heated to 115°; let sit until foamy, about 10 minutes. Add flour and salt, and mix on low speed until dough forms; increase speed to medium, and knead dough until smooth, about 8 minutes. Cover bowl with plastic wrap, and let sit until doubled in size, about 1 ½ hours.
Meanwhile, make the filling: Heat oil in a 12″ skillet over medium-high heat; add garlic and onions, and cook, stirring occasionally, until deeply caramelized, about 30 minutes. Remove from heat, and stir in poppy seeds and salt and pepper; set aside to cool.
Uncover and punch down dough; cover again and let sit until doubled in size again, about 1 hour. Uncover dough and transfer to a clean work surface; portion and shape into about eighteen 2-oz. balls. Place 6 balls each on 3 parchment paper-lined baking sheets, spaced evenly apart; cover with plastic wrap and let sit until puffed, about 30 minutes. Uncover balls, and using the palm of your hand, gently flatten each into a disk; cover again and let sit until puffed, about 30 minutes.
Heat oven to 450°. Uncover balls, and, using your fingers, press the center of each to indent; continue pressing and stretching center of each dough ball until you're left with a thin center membrane surrounded by a thick ring of dough on the outer edge. Fill centers of each dough round with about 1 tsp. onion-poppy seed filling. Working with one baking sheet at a time, place in oven, and spray bialys with water until completely coated. Bake until lightly browned and still soft, about 16 minutes.