Place 16 oz (about 2½ cups) raw buckwheat groats in a large bowl and cover with cool water (2 inches above). Soak 8-12 hours or overnight until the groats are soft and slightly gelatinous.
Pour the soaked buckwheat into a fine-mesh strainer. Rinse thoroughly under cold water for 30-45 seconds, gently rubbing to remove excess gel, then drain completely.
Transfer drained groats to a high-speed blender. Add ½ cup fresh water and blend 60-90 seconds, scraping sides as needed, until smooth and thick like pancake batter.
In a large bowl, fold the blended buckwheat with ¼ cup honey, ¼ cup ground flaxseed, ½ tsp. salt, and 1 tbsp. baking powder until fully combined. Batter should be thick but pourable.
Line an 8.5 x 4.5-inch loaf pan with parchment (leaving overhang) or grease well. Pour in batter, smooth the top, and sprinkle desired toppings, pressing lightly to adhere.
Bake at 375°F (190°C) for 1 hour 15 minutes, until golden and a toothpick comes out mostly clean. Cool 15 minutes in the pan, then transfer to a wire rack for at least 1 hour before slicing to allow the loaf to set.
Notes
Don't Skip The Soak: Unsoaked buckwheat won't blend properly and creates a gritty texture. The 8-hour minimum activates enzymes crucial for the bread's structure.
Use Raw, Not Roasted: Roasted buckwheat (kasha) has been heat-treated, destroying the starches needed for binding. Only raw groats work in this recipe.
Blend Until Smooth: A few small specks are fine, but grainy batter produces dense, crumbly bread. Blend for a full 90 seconds if needed.
Don't Overmix After Adding Baking Powder: Fold just until combined. Overmixing deflates the batter and creates a dense loaf.
Cool Completely Before Slicing: Cutting warm bread causes it to crumble and compress.