While the pre-ferment is rising, toast the sesame seeds in a skillet, tossing and mixing until they are two shades darker (3-5 min). Let them cool.
Also toast the sunflower seeds, while tossing them around and let them cool
Toast the flax seeds until they start to pop and let them cool down
Grind the flax seeds to meal in a mortar and pestle and mix them with the flour
Grind the buckwheat in the mortar and pestle and add them to the flour mixture
Mix all the other dry ingredients including the cooled sesame seeds in the flour mixture in a large enough bowl, that can also accommodate all the water, all the starter mixture and then still allows for the dough to rise
About an hour before the starter reaches its top, add the water to the flour/nut and seed mixture and mix well. The autolysis due to the wetting of the mixture will help the dough to become more extensible.
Add the sourdough starter when ready and start to mix with the spatula or a fork. You can do this by taking a part of the dough mixture with your fork or spatula to the other side of the bowl. The dough should stretch like chewing gum in this exercise. Repeat this for a few minutes until all is mixed well.
Now the main -bulk- fermentation will happen. As to build enough gluten strength to this dough, every 20-30 minutes, make a full fold of the dough. A full fold is starting north with wetted fingers grabbing the dough and stretching it up and then folding it towards you in the south direction. Then turn the bowl 180 degrees and repeat (so you are stretching the dough again and folding it back). Then turn the bowl 90 degrees and repeat. And then turn the bowl 180 degrees and repeat again. That is one full fold.
You will see the dough rise and with every fold it becomes more difficult to complete the last part of the fold completely. When the dough has risen 2-2.5 times in volume, shape the dough
I use a tea towel in a loaf pan that has been lightly sprinkled with some flour. I fold the dough carefully with floured fingers, so it can nicely fit in the loaf pan. keep the fold wrinkles up on top.
You can let it proof for a little while more at room temperature until the dough has risen sufficiently for a 2-2.5 volume increase (from the original volume). Then cover with some plastic wrap, oiled with a little oil. Retard in the refrigerator, where the rising will slow down. Bake the bread the next day