Combine 500g bread flour, 350g water at 75 to 80°F, 100g active sourdough starter, and 5g sugar in a large mixing bowl and mix for 3 to 4 minutes until a shaggy dough forms.

Cover and let the dough rest (autolyse) for 30 minutes at room temperature.

Sprinkle 10g salt and 7g instant yeast over the dough and mix on medium speed for 5 to 7 minutes (or knead by hand for 10 minutes) until smooth and elastic.

Place the dough in a lightly oiled bowl and bulk ferment at room temperature (around 75°F) for 4 to 6 hours, performing stretch and folds every 30 minutes for the first 2 hours.

Turn the dough onto a floured surface, divide into 8 equal pieces (about 110g each), and shape each into a tight ball.

Place shaped buns seam-side down on a parchment-lined sheet, cover, and cold ferment in the refrigerator for 8 to 12 hours.

Remove the buns from the refrigerator and let them rest at room temperature for 45 to 60 minutes while preheating the oven to 450°F (preheat Dutch oven inside for at least 30 minutes if using).

Brush each bun with egg wash (1 egg beaten with 1 tablespoon water) and sprinkle with sesame seeds or everything bagel seasoning.

Make a shallow slash on the top of each bun, then bake covered in a preheated Dutch oven at 450°F for 15 minutes (or bake on a sheet pan with steam by spraying water twice during the first 10 minutes).

Remove the lid and bake for an additional 8 to 12 minutes at 450°F until deep golden brown and the internal temperature reaches 205 to 210°F. Cool completely on a wire rack before slicing.