Preheat oven to 350°F (175°C). Grease a loaf pan or line it with parchment paper.
In a large mixing bowl, combine buckwheat flour, baking soda, salt, and flaxseed. Mix well.
In a separate bowl, whisk water, apple cider vinegar, and olive oil. Pour into the dry ingredients.
Stir until fully combined. The batter will be thick but spreadable. If using seeds, fold them in now.
Pour the batter into the prepared loaf pan and smooth the top with a spatula.
Let the batter rest for 20–30 minutes at room temperature to help the flour absorb moisture.
Bake for 55–60 minutes, or until a toothpick inserted in the center comes out clean.
Let cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before slicing.