In a large bowl, combine your sourdough starter, warm water, and sugar. Gradually add the flour and salt, mixing until a dough forms. Knead the dough on a floured surface for about 8-10 minutes until smooth and elastic. If using your stand mixer, use the dough hook attachment and knead on level 2 for about 5 minutes until the dough forms a ball. You want your dough to be stiff but not dry.

Transfer the dough to a large bowl, cover with a damp cloth or plastic wrap, and let it rise in a warm spot for 8-12 hours, or until it has doubled in size. This time may vary depending on the strength of your starter and room temperature.

Once the dough has risen, divide it into 10-12 equal portions. Roll each portion into a ball, then use your thumb to poke a hole in the center, gently stretching it to form a bagel shape. Place the shaped bagels on a parchment-lined baking sheet and cover with a cloth. Let them rest for about 1 hour.

Preheat your oven to 425°F (220°C). Bring a large pot of water to a boil and add baking soda and brown sugar. Carefully drop in the bagels, 2-3 at a time, boiling for 30 seconds on each side. Remove them with a slotted spoon and place back onto the baking sheet. Brush with egg wash and sprinkle with your choice of toppings. Bake for 15-20 minutes, or until golden brown.