1. Prepare the Dough Base: In a large mixing bowl, combine the active sourdough starter, warm water, granulated sugar, and salt. Stir until the sugar and salt are dissolved and the starter is well incorporated. This forms the liquid foundation for your dough.

2. Mix and Knead the Dough: Gradually add the bread flour to the wet ingredients, mixing with a wooden spoon or your hands until a shaggy dough forms. Turn the dough out onto a lightly floured surface and knead for 8-10 minutes until it becomes smooth, elastic, and no longer sticky. It should spring back slightly when poked.

3. First Rise (Bulk Fermentation): Place the kneaded dough in a lightly oiled bowl, turning it once to coat. Cover the bowl with plastic wrap or a damp kitchen towel and let it rise at room temperature for 8-12 hours, or until doubled in size. This is often best done overnight.

4. Shape the Bagels: Gently punch down the risen dough and divide it into 8-10 equal pieces. Roll each piece into a smooth ball. Using your thumb, press a hole through the center of each ball and gently stretch the dough to create a bagel shape with a hole about 1-1.5 inches in diameter. Place the shaped bagels on a parchment-lined baking sheet.

5. Boil the Bagels: Preheat your oven to 425 F (220 C). Bring a large pot of water to a rolling boil and add the baking soda. Carefully drop 2-3 bagels into the boiling water at a time, ensuring they don’t overcrowd the pot. Boil for 30 seconds per side, then remove with a slotted spoon and return them to the parchment-lined baking sheet.

6. Apply Cinnamon Sugar Topping and Bake: In a shallow dish, combine the 1/2 cup granulated sugar and 2 tablespoons ground cinnamon. Brush the boiled bagels generously with melted butter, then immediately dip each side into the cinnamon-sugar mixture, ensuring they are well coated. Place them back on the baking sheet.

7. Bake to Golden Perfection: Bake the bagels for 20-25 minutes, or until they are golden brown and sound hollow when tapped on the bottom. The cinnamon sugar should be slightly caramelized and fragrant.

8. Cool and Serve: Transfer the baked bagels to a wire rack to cool slightly before serving. They are best enjoyed warm, fresh from the oven.