1. Mix the Dough: In a large mixing bowl, combine the active sourdough starter, warm water, sugar, and salt. Stir until the starter is mostly dissolved. Gradually add the bread flour, mixing with a wooden spoon or your hands until a shaggy dough forms.

2. Knead the Dough: Turn the dough out onto a lightly floured surface. Knead for 8-10 minutes until the dough is smooth and elastic. It should pass the windowpane test (stretch a small piece thin enough to see light through it without tearing). Place the dough back into a lightly oiled bowl, cover, and let it bulk ferment at room temperature for 3-4 hours, or until it has visibly increased in size.

3. Shape the Bagels: Gently deflate the dough and divide it into 8 equal pieces (about 110-120 grams each). Roll each piece into a smooth ball. To create the hole, poke a hole in the center of each ball with your thumb and gently stretch the dough into a ring, aiming for a consistent thickness. Place the shaped bagels on a parchment-lined baking sheet, cover loosely with plastic wrap, and refrigerate for 8-12 hours (overnight). This cold proof develops flavor and makes them easier to handle.

4. Prepare for Baking: Preheat your oven to 425 F (220 C) with a baking stone or inverted baking sheet inside. Bring a large pot of water to a rolling boil and add the baking soda.

5. Boil the Bagels: Carefully drop 2-3 bagels into the boiling water, making sure not to overcrowd the pot. Boil for 30-60 seconds per side. The longer they boil, the chewier the final texture will be. Use a slotted spoon to remove the bagels and place them back on the parchment-lined baking sheet.

6. Add Toppings and Bake: If desired, sprinkle your chosen toppings onto the wet bagels immediately after boiling. Transfer the baking sheet to the preheated oven. Bake for 18-25 minutes, or until the bagels are deeply golden brown and sound hollow when tapped on the bottom.

7. Cool and Serve: Allow the bagels to cool completely on a wire rack before slicing and serving. This helps the internal structure set and prevents a gummy texture.