01 - Heat olive oil in a skillet over medium-low heat. Add sliced onions and salt. Cook, stirring occasionally, until deeply caramelized and golden brown, about 20 to 25 minutes. Set aside to cool completely before using.
02 - In a large bowl, combine the sourdough starter, warm water, and honey. Stir until fully dissolved. Add bread flour, whole wheat flour, and salt. Mix with a wooden spoon until a rough, shaggy dough forms.
03 - Turn the dough onto a lightly floured surface. Knead firmly for 8 to 10 minutes, pushing the dough away with the heel of your hand and folding it back over itself, until smooth, elastic, and no longer sticky.
04 - Place the dough in a lightly oiled bowl, turning once to coat all sides. Cover with plastic wrap or a damp towel. Let rise at room temperature for 4 to 6 hours, or until doubled in size.
05 - Turn the dough onto a floured surface and divide into 8 equal portions. Shape each piece into a tight ball by tucking the edges underneath. Poke a hole through the center with your finger and gently stretch to form a 2-inch ring. Transfer to a parchment-lined baking sheet.
06 - Cover the shaped bagels loosely with plastic wrap. Let proof at room temperature for 1 hour, then refrigerate overnight for 8 to 12 hours to develop flavor and texture.
07 - Preheat oven to 425°F. In a large saucepan or Dutch oven, bring 2 quarts of water and barley malt syrup to a gentle boil over medium-high heat.
08 - Remove bagels from the refrigerator. Working in batches, boil each bagel for 45 seconds per side, using a slotted spoon to gently lower them into the water and transfer them back to the baking sheet.
09 - Immediately press the cooled caramelized onions onto the tops of the boiled bagels, pressing gently to adhere. Bake for 20 to 25 minutes, rotating the baking sheet halfway through, until golden brown and crisp.
10 - Transfer bagels to a wire rack and let cool for at least 30 minutes before slicing. This prevents the interior from becoming gummy and ensures the best texture.