In a large bowl add the starter and water and mix to combine. Add the vital wheat gluten, whole wheat flour, all-purpose flour, and salt and mix to combine, just a couple of minutes. Cover loosely and let rest an hour.
MEANWHILE, in a small bowl add the white sesame seeds, the flax seed, and the fennel seeds and pour the hot water over. Cover and let rest while the dough is resting.
After the aforementioned rest for an hour, mix the dough over onto itself, turn to stretch and fold. Add about half the seed soak and half the sunflower and (if using) half the nigella seeds. Do stretches an folds until combined, then add the rest of the seeds and continue stretching and folding. NOTE: if it seems dry or too wet, add water or flour a teaspoon at a time. A lot of this hydration will depend on how much water you added to the seeds, seasonal humidity, etc.
Every 30 minutes for the next two hours do 10-20 stretch and folds then cover and let rest. A bit of olive oil will help reduce sticking when doing the stretch and folds. (See notes.)
At the end of 2 hours and after your last stretches and folds, cover and let rest at room temperature or in the refrigerator until ~50% larger.
Prepare your two bannetons with all-purpose and/or rice flour.
Remove dough to a floured surface and gently punch down. Divide in two (should be about 660 grams each loaf) and fold and shape into a round. Sprinkle the top with the black sesame seeds and gently press them into the loaf. Turn upside down into banneton (I sprinkled a few seeds into the banneton, too). Repeat with the other loaf, cover, and let both rest for 30 minutes.
Pop into the refrigerator and let ferment overnight or up to two nights. This bulk ferment phase really helps with the development of the sour flavor.