In a large mixing bowl, mix together the sourdough starter and 200g of water. Add the bread flour and mix until a shaggy dough forms. Cover and let sit for 30min to 1 hour
Add the salt and 5g of water to the dough. Gently use your hands to fold in the salt. Cover and let sit for 30 - 45 min
Perform your first coil fold. Wait 30 - 45 minutes more and perform your second coil fold.
Mix together the sesame seeds and pumpkins seeds. Then, 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. Sprinkle the surface generously with the seed mixture. Fold the bottom edge of the dough towards the middle and sprinkle the exposed side with more seeds. Fold in the sides toward the middle, then fold down the top to create a rough ball. Place the dough back in the bowl and sprinkle with any of the remaining seeds. Cover and let sit for another 30 - 45 minutes.
Perform the last coil fold, cover, and let sit for another 2 - 3 hours
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 and place it seam side up in the bowl. Let sit for another 2 - 3 hours. Periodically check on the dough to make sure it does not overproof.
Cover with plastic wrap and 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. Turn the dough onto floured parchment paper and 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 20 – 22 minutes until it makes a hollow noise when tapped.
Remove from oven and let cool completely (about 2 hours) before slicing