Add your gluten free sourdough starter, flour, water, and salt to a large bowl.
Cover with a towel and let sit out on the counter overnight to rise.
In the morning, add the additional flour and mix the dough. You will need to use a little judgment to see how stiff your dough is. See the note below on gluten free flour blends.
Line a baking sheet with parchment paper.
Shape your bagels either by forming a circle and poking a finger in the middle or roll your dough into thick rows then merge the ends to form bagel shapes. *Note, depending on if you use a sourdough starter or dry yeast, you WILL need to adjust flour amounts to compensate for the liquid. Please add flour until your dough is similar in consistency to the photo in my post for Step 2.
Cover the bagels gently with plastic wrap so they don't dry out.
Allow the bagels to rise 1-2 hours (depending on how warm it is on your counter.)
Preheat the oven to 425º F.
Bring 4 cups of water and 3 tablespoons maple syrup to boil in a small pot.
Drop a bagel gently into the boiling water and cook 25 seconds on each side.
Remove the bagel with a slotted spoon and allow water to drain.
Place the bagels onto a parchment paper-lined baking sheet.
Brush each bagel with egg whites. Add optional toppings.
Bake for 20-25 minutes depending on bagel size and thickness.
Bagels will be lightly golden brown when finished baking.