In a large mixing bowl, mix together your starter plus 215g of water. Stir to disperse. Add the flour and mix with your hands until a shaggy dough forms. Cover and let sit for 30 minutes.
Add the salt and remaining 10g of water to the dough. Use your hands to incorporate. The dough may look like it's coming apart, but keep going until everything is incorporated. Cover and let sit for 30 minutes
Gently slide your hand under the middle of the dough and fold it over itself twice using the coil fold method. This helps gently develop the gluten. Cover and let sit for another 30 minutes.
Add the sesame seeds to the dough and perform another set of coil folds. The seeds will naturally incorporate into the dough during the folding process. Cover and let sit for another 30 minutes.
Laminate your dough: sprinkle a working surface with water to prevent sticking. Turn your dough onto the surface and very gently stretch until thin, without breaking the dough. It should be about 8 – 10 inches long on each side. Use your fingers to gently spread the red bean paste evenly across the surface of the dough. Roll up the bottom of the dough towards the middle. Take the left edge of the dough and bring it to the middle. Then, take the right edge and bring it over to the left. Fold down the dough from the top to create a rough dough ball. Place the ball back in the bowl. If the dough is loose enough, perform another set of coil folds. Cover and let sit for another 30 - 45 minutes.
Line a medium bowl with a floured kitchen or paper towel. Lightly flour a working surface and turn the dough onto the surface. Gently shape into a boule. The dough will be quite sticky, so be gentle with the shaping. The dough may rip a little and the red bean could come through - that's okay. Place the dough seam side up in your floured bowl.
Cover and let rise for another 45 minutes to 1 hour. The dough should feel airy and jiggle a little when you shake the bowl. Place in the fridge for at least 8 hours and up to 48 hours.
When you're ready to bake, preheat the oven to 500°F with your uncovered Dutch oven inside. Take the loaf from the fridge and sprinkle with flour. Turn the dough onto floured parchment paper. Spritz or lightly brush the surface of the loaf with water and cover with black sesame seeds, then score.
Place the scored loaf in the Dutch oven and cover with the lid. Bake for 25 minutes. Remove the lid and reduce temperature to 450°F and bake for another 12 – 15 minutes until a rich deep brown and hollow when tapped.
Remove from oven and let cool completely (about 2 hours) before slicing.