Mix together sourdough starter, warm water, sugar, flour, and sea salt in stand mixer and knead until a smooth dough forms, about 5-6 minutes.
Shape into a ball and let it rest in a lightly oiled bowl. Cover and place at room temperature (about 21°C) for 8-10 hours, until slightly puffy and doubled in size. Transfer into the fridge to rest overnight.
On the day you want to make:
In the morning, remove dough from the fridge and let it rest at room temperature for 3-4 hours. Dough should be slightly puffy.
Divide dough into 10 pieces and shape into a bagel. (You can roll into a log and join the ends together, or simply make a round and use your thumb to poke a hole in the middle).
Place dough onto a large parchment-lined baking sheet. Let it rest for 15-20 minutes.
Preheat oven to 425°F/218°C.
In the meantime, bring 8 C water to a boil and add in honey. Lower to a simmer.
Place the dough into the simmering liquid and boil for about 30 seconds, until the bagel floats. [Don't let it sit in the hot water for more than 30 seconds or else the bagel will not rise very well].
Remove with a slotted spoon and add toppings (if using).
Place the bagel topping side up onto the baking sheet. Repeat with the remainder.
Bake for 20-25 minutes, until slightly golden in colour. Best enjoyed while fresh.
Note: Bagels can be sliced in half and frozen in an airtight container for up to 3 months. Pop into the toaster before serving.