Prepare Starter (1-2 Days Before Baking):

1. Feed your sourdough starter using a 1:1 ratio of water to flour by weight to get 3/4 cup. 1/2 will be used for this bread, 1/4 as your next starter. Use filtered water and any available flour (rye, white, or whole wheat).

Mix the Dough (Evening or Morning Before Baking):

1. In a jug, mix wet ingredients: sourdough starter, water, malt extract, and diastatic malt (if using).
2. In a separate large bowl, combine dry ingredients: bread flour, rye flour, buckwheat flour, salt, vitamin C powder (if using), and 3 tbsp sesame seeds.
3. Gradually stir the wet mixture into the dry ingredients until all flour is hydrated and no dry spots remain. A sturdy wooden spoon works well.

Bulk Fermentation:

1. Cover the bowl with cling film and let rest at room temperature for 2 hours.
2. Afterward, place the dough in the refrigerator overnight (preferred for flavor) or allow it to ferment on the counter for at least 6 hours until doubled in size and puffy.

Shape the Dough:

1. Remove dough from the fridge 2 hours before baking to reach room temperature.
2. Lightly flour baking parchment. Gently stretch and fold the dough: stretch one edge and fold it inward, then the opposite side, rotate 90 degrees, and repeat 2 more times.
3. Flip the dough seam-side down and shape into a smooth loaf by tucking in edges.

Add Sesame Topping (Optional):

1. Brush the loaf with water and roll the top in sesame seeds spread on a plate. Coat generously.

Final Proof:

1. Transfer the loaf (on parchment) to a banneton or bowl. Cover with a towel and let rise for 1 hour.

Preheat the Oven:

1. Preheat oven to 450°F (230°C) with a Dutch oven or cast-iron pot inside. Allow the pot to heat for at least 30 minutes.

Score and Bake:

1. Optionally, score the bread with a sharp knife or scoring tool for decorative patterns.
2. Carefully transfer the dough (on parchment) into the hot Dutch oven. Trim excess parchment corners to retain steam.
3. Bake covered for 30 minutes, then remove the lid and bake for 15-25 minutes at 425°F (220°C) until the crust is golden brown.

Cool and Enjoy:

1. Remove the bread from the oven and let cool on a rack for at least 1 hour before slicing. This allows the crumb to set and flavors to develop.