Whisk water, sourdough starter, and 17 grams of molasses in a medium sized bowl until combined.
In a separate large bowl, combine bread flour and salt. Pour the wet ingredients into the flour and combine using your hands or a bowl scraper, just until a shaggy dough forms.
On a clean, un-floured surface, remove the dough from the bowl. Begin kneading the dough by hand for a full 15 minutes (don’t cut this time short). You'll know the dough is done kneading when it is flexible and bouncy to the touch. To check that you've kneaded long enough, grab a piece of dough between your fingers, stretch it, and see the web fibers that have developed. You should be able to pull the dough thin enough that light shines through. If your dough rips as soon as you start pulling, you need to keep kneading the dough. You want the dough to be able to stretch rather than tear.
Place the dough in a clean glass bowl and cover with plastic wrap. Allow the dough to rise in a warm area until it’s doubled in size, about 7-8 hours. The time it takes to double in size can vary depending on the climate and temperature of your house.
While the dough is rising, prepare two baking sheets generously sprinkled with cornmeal. Set aside.
Gently punch down the risen dough a few times to release some of the gas that formed. On a clean, un-floured surface, remove the dough from the bowl. Using a bench scraper, divide the dough into pieces weighing about 107g each.
Working with one piece of dough at a time, and keeping the others covered with a tea towel, roll the dough into a ball. Push through the center of the ball using both thumbs to make a hole. Then, continue to stretch the bagel out until it reaches about 4 inches (10 cm) across. Be sure not to flatten the bagel too much throughout this process. Place completed bagels about 2 inches (5 cm) apart on the previously prepared baking sheets, and cover with plastic wrap. Repeat until all pieces of the dough have been shaped.
Securely cover the bagels with plastic wrap, so no air can get into the baking sheet. Allow dough to proof overnight in the refrigerator for 12–18 hours.
Arrange the oven racks to the upper and lower thirds of the oven. Place a pizza stone on each of the racks, then heat oven to 450°F (230°C). Allow the oven and pizza stones to preheat for at least 30 minutes.
Fill a large pot or dutch oven about halfway full with water. Place over high heat to boil. Once the water is at a rolling boil, add in molasses, a tablespoon at a time, until the water is dark brown.
Remove the bagels from the refrigerator. They will have puffed up slightly overnight. Gently, place as many bagels into the boiling water as will comfortably fit without over crowding the pot or dutch oven. Allow the bagels to boil for 30 seconds on the first side, then flip, and boil for an additional 30 seconds on the second side. Using a slotted spoon, remove the bagel from the boiling water and place on a wire rack to cool. Repeat until all of the bagels have been boiled.
Carefully, remove the hot baking stones from the oven. Transfer the cornmeal from the original baking sheets to the hot pizza stone to prevent sticking. Place bagels on the stone about 2 inches apart. Bake for 22 - 25 minutes, rotating shelves halfway through until bagels are deeply browned and shiny on top. Keep an eye on your bagels (without opening the oven door) to ensure they're not getting too dark. Remove from the oven, and let cool completely on a wire rack.