Soak the buckwheat and quinoa. Add the buckwheat and quinoa into a large bowl, cover them with water, and let them soak for 4-6 hours (ideally, no longer than 8 hours) at room temperature. Then, drain the soaking water and rinse the seeds under lukewarm running water.
Sprout the buckwheat and quinoa (optional). Transfer the soaked buckwheat and quinoa to a strainer and set the strainer over a large bowl to drain any excess water. If using a sprouting jar, invert it into a large bowl or other object that allows it to sit at an angle. After 8-12 hours, sprouts should start to emerge. If there are no sprouts, drain, rinse, and leave the seeds sprouting for 6-8 more hours. While sprouting is not necessary, it does boost fermentation activity.
Blend. Add the sprouted (or just soaked) buckwheat and quinoa to a high-speed blender together with water and blend until smooth. The batter might be slightly gritty, but no seeds should be visible.
Ferment. Transfer the batter to a clean bowl, cover it with a cheesecloth, and let it ferment for about 14 hours at 80°F/27°C. The fermentation time will depend on the temperature - the fermentation rate accelerates as temperature increases. You are looking for the volume of the batter to increase by about a third and for lots of air bubbles on top and throughout the batter.
Proof. At the end of the fermentation, gently mix in the salt, preserving as much volume and air bubbles as possible. Pour the batter into a parchment paper-lined 8 x 4 inch/20 x 10 cm loaf pan and let it rise for 30-60 minutes. Ensure you don't over-proof the bread; otherwise, the center will collapse during baking.
Bake. Slide the loaf pan into the oven and bake the bread at 350ºF/177ºC until the bread pulls away from the sides of the loaf pan and the crust turns golden brown, for about 90 minutes.
Cool. Transfer the bread onto a cooling rack and let it cool completely before slicing. Cutting fresh-baked bread too early will result in a gummy and sticky interior.
Store. Wrap the bread in a large tea towel (so it can breathe) and store it in a cool place for up to 3 days. For longer-term storage, slice the bread first and then transfer individual slices into an airtight bag, one on top of the other, in an alternating 90-degree pattern. Freeze for up to 3 months.