Feed your starter. Feed your sourdough starter 8 hours before you plan on using it. In a jar or bowl, stir together 30 grams of sourdough starter, 45 grams of all-purpose flour, and 45 grams of water. (You can double these amounts if you want more to feed later). Cover with plastic wrap or a tea towel and allow your mixture to sit on your counter for 8 hours or until it is bubbly and has doubled in size.
Mix starter, water, sugar. Add the warm water, active sourdough starter, and sugar in a large mixing bowl. Stir until the water and sourdough starter is almost fully incorporated.
Add flour and salt. Add the flour and salt and use a wooden spoon to combine as much as you can. You may need to use your hands to add the rest of the flour.
Rest. Place the formed dough in a clean bowl and cover with a towel, plate, or plastic wrap. Allow to sit covered for 1 hour.
Add cinnamon and raisins. After 1 hour, remove the cover. Turn the dough onto a clean, dry surface (if the dough is sticky, sprinkle a small amount of flour to the surface). Sprinkle half of the cinnamon and raisins onto the dough. Fold the dough into thirds, covering the cinnamon and raisins. Then sprinkle the rest of the cinnamon and raisins onto the dough and roll the dough up into a ball.
Bulk rise. Transfer the dough back to the bowl. Cover with a towel, plate, or plastic wrap and allow to rise for 8-12 hours or overnight.
Divide dough. Remove the cover and transfer the dough to a clean, dry space using a sprinkle of flour as needed. Divide the dough into 12 equal portions and form them into a ball.
Rest. Cover with a towel and allow to sit for 1 hour.
Prep water bath. Prepare the water bath by adding the water, brown sugar, and baking soda (if using) to a large pot and boil over medium-high heat.
Prep parchment paper. While the water is coming to a boil, cut a sheet of parchment paper down into 12 squares 4x4 inch (2.5x2.5 cm) and set aside.
Prep for baking. Preheat the oven to 375°F (190 C) and line 2 baking sheets with parchment paper. Set aside.
Prep dough. Uncover the dough balls and use your thumb to press a hole into the middle of each dough ball, stretching it slightly. Place each prepared bagel onto a piece of the 4x4 inch (2.5x2.5 cm) parchment square. Repeat this process 11 more times until all of the dough balls have been shaped into bagel shapes. (Note: The parchment paper allows for easy transfer, preventing the dough from sticking to your hands or the counter.)
Boil. Working in batches, place the bagels into the water bath. Allow them to cook for 3 minutes, flipping halfway. Transfer the boiled bagels to the parchment-lined baking sheet using a spider or slotted spoon.
Bake. Bake at 375°F (190 C) for 25 minutes or until golden brown.
Cool. Remove from the oven and allow them to cool on the pan for 5 minutes before transferring to a wire cooling rack. Allow them to cool on the cooling rack for 10 minutes before cutting into them. Serve warm or at room temperature.