01 - Heat olive oil in a skillet over medium-low heat. Add sliced onions and 1/2 teaspoon salt. Cook, stirring occasionally, until deeply caramelized and browned, about 20 to 25 minutes. Remove from heat and set aside to cool completely.
02 - In a large mixing bowl, combine the active sourdough starter, warm water, and honey. Stir thoroughly until the honey dissolves completely into the mixture.
03 - Add bread flour, whole wheat flour, and sea salt to the bowl. Mix until a rough dough forms. Turn onto a lightly floured surface and knead for 8 to 10 minutes until the dough becomes smooth, elastic, and passes the windowpane test.
04 - Place the kneaded dough in a lightly oiled bowl, cover with plastic wrap or a damp towel, and let rest at room temperature for 4 to 6 hours 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 opening, creating the classic bagel shape.
06 - Arrange shaped bagels on a parchment-lined baking sheet. Cover loosely with plastic wrap and let proof at room temperature for 1 hour. Refrigerate overnight for 8 to 12 hours to develop flavor and texture.
07 - Preheat oven to 425°F. Ensure the oven rack is positioned in the center for even baking.
08 - Bring 8 1/2 cups of water and 1 tablespoon barley malt syrup (or honey) to a gentle boil in a large saucepan or Dutch oven.
09 - Remove bagels from the refrigerator. Working in batches, boil each bagel for 45 seconds per side, using a slotted spoon to gently flip them. The bagels should float during this process.
10 - Return boiled bagels to the parchment-lined baking sheet. Immediately press the cooled caramelized onions onto the tops of the bagels while they are still tacky from boiling.
11 - Bake for 20 to 25 minutes until the bagels achieve a deep golden-brown color with a crisp exterior. Rotate the baking sheet halfway through for even browning.
12 - Transfer baked bagels to a wire rack and let cool completely before slicing. This prevents the interior from becoming gummy and ensures proper texture development.