01 - Heat olive oil in a skillet over medium-low heat. Add sliced onions and 0.5 tsp salt. Cook, stirring occasionally, until deeply caramelized and golden brown, approximately 20–25 minutes. Remove from heat and set aside to cool completely.
02 - In a large mixing bowl, combine sourdough starter, warm water, and honey. Stir thoroughly until starter is fully dissolved and mixture is uniform.
03 - Add bread flour, whole wheat flour, and sea salt to the liquid mixture. Mix with a wooden spoon until a rough, shaggy dough forms. Turn onto a lightly floured surface.
04 - Knead dough by hand for 8–10 minutes until smooth, elastic, and passes the windowpane test. Dough should feel tacky but not sticky.
05 - Place dough in a lightly oiled bowl, turning to coat all sides. Cover with plastic wrap or damp towel. Let ferment at room temperature for 4–6 hours until doubled in size.
06 - Turn dough onto floured surface and divide into 8 equal portions. Roll each piece into a tight ball by tucking edges underneath. Poke thumb through center, then gently stretch to form a 2-inch hole, creating even ring shape.
07 - Arrange shaped bagels on parchment-lined baking sheet. Cover loosely with plastic wrap. Let proof at room temperature for 1 hour, then refrigerate overnight for 8–12 hours to develop flavor.
08 - Preheat oven to 425°F.
09 - Bring 8 cups water and barley malt syrup (or honey) to a gentle boil in a large saucepan or Dutch oven.
10 - Remove bagels from refrigerator. Working in batches, boil bagels for 45 seconds per side. Using slotted spoon, transfer back to parchment-lined baking sheet, draining excess water.
11 - Immediately press cooled caramelized onions onto the tops of boiled bagels, ensuring even coverage and good adhesion.
12 - Bake for 20–25 minutes until bagels are deep golden brown with crispy exteriors. Rotate baking sheet halfway through for even browning.
13 - Transfer baked bagels to wire rack. Let cool completely for at least 30 minutes before slicing to preserve texture.