In a mixing bowl, combine the bread flour, whole wheat flour, salt, and sugar. Whisk to combine.
Add the active sourdough starter and water. Mix with a wooden spoon or your hands until a shaggy dough forms.
Knead the dough on a floured surface for about 10 minutes until smooth and elastic.
Place the kneaded dough into a lightly greased bowl, cover it with a damp cloth, and let it rest at room temperature for about 2 hours, or until it has doubled in size.
After the resting period, gently deflate the dough and divide it into 8 equal pieces. Shape each piece into a ball.
Poke a hole in the center of each ball and gently stretch it to form a bagel shape. Place the shaped bagels on a parchment-lined baking sheet.
Cover the bagels with a damp cloth and allow them to rise for another 30 minutes.
Preheat your oven to 425°F (220°C).
While the oven heats, bring a large saucepan filled with water to a boil. Add the baking soda.
Boil the bagels, 1 or 2 at a time, for about 30 seconds on each side. Use a slotted spoon to transfer them back to the baking sheet after boiling.
If desired, sprinkle with your choice of toppings right after boiling.
Bake in the preheated oven for 20-25 minutes, or until the bagels are golden brown.
Remove from the oven and let cool on a wire rack before serving.