01 - Whisk together active sourdough starter, lukewarm water, and sugar in a large mixing bowl until sugar dissolves completely.
02 - Add bread flour, whole wheat flour, and sea salt to the starter mixture. Mix with a wooden spoon until shaggy dough forms and no dry flour remains.
03 - Turn dough onto lightly floured surface. Knead for 8 to 10 minutes until smooth and elastic. Drizzle vegetable oil onto dough during final 2 minutes of kneading, working until fully absorbed.
04 - Gently fold raspberries into dough, handling minimally to keep berries whole. Dough will appear sticky with red marbling throughout.
05 - Place dough in greased bowl, cover with plastic wrap or damp towel. Let rise at room temperature for 6 to 8 hours until doubled in size.
06 - Turn risen dough onto floured surface. Divide into 8 equal portions using a dough scraper or knife.
07 - Form each portion into a smooth ball. Poke thumb through center to create hole. Gently stretch and rotate to form even ring with 2-inch diameter opening.
08 - Arrange bagels on parchment-lined baking sheet. Cover with clean towel. Let proof at room temperature for 1 to 2 hours, or refrigerate overnight for deeper sourdough flavor.
09 - Preheat oven to 425°F. Position oven rack in center position.
10 - Bring 12 cups water to rolling boil in wide pot. Stir in honey and baking soda if using until dissolved.
11 - Working in batches, boil bagels for 45 to 60 seconds per side. Remove with slotted spoon, allowing excess water to drip off. Return to parchment-lined baking sheet.
12 - Immediately sprinkle boiled bagels with Demerara sugar or poppy seeds while surface remains wet, if desired.
13 - Bake for 22 to 25 minutes until deeply golden brown and crusty. Rotate baking sheet halfway through for even coloring.
14 - Transfer baked bagels to wire rack. Cool for at least 30 minutes before slicing to prevent gummy texture.