Preheat the oven to 160°C. Lightly grease a 23 x 10cm loaf tin with olive oil.
Blend the soaked buckwheat in batches in a high-speed food processor, to finely grind to a porridge-like consistency. Transfer into a large bowl. Add the water, olive oil, chia seeds, ground flaxseed, honey, apple cider vinegar, bicarbonate of soda and salt. Mix well to combine. Set aside for 10 minutes, for the chia and ground flaxseed to hydrate, to make a thick batter.
Combine the amaranth, quinoa, pumpkin and sunflower seeds in a medium bowl. Set aside ½ cup of the seed mixture. Add the remaining mixture to the buckwheat batter and stir to combine. Pour the batter into the prepared tin. Scatter the reserved seeds over the top.
Bake for 2 hours, until golden brown and the loaf sounds hollow when tapped on the base. Leave in the tin for 15 minutes, to cool slightly.
Turn out onto a rack to cool completely. Slice to serve.
