Preheat oven to 200°C/375°F and line two baking sheets with baking parchment.
For the dough mix buckwheat flour, salt, flax seeds and baking powder in a bowl. Add the olive oil and water and knead into a flexible dough. Roll into a ball. Keep aside.
For the topping heat the olive oil in a pan and add onions and apple slices. Season with salt and black pepper. Fry until onions are translucent.
Prepare the vegan sour cream by mixing the ground cashew nuts with apple cider vinegar and water. Season with salt and black pepper. Mix until you have a creamy consistency.
Divide the dough into four parts and roll out each onto a floured surface. Spread some sour cream on top and add the apples and onions. Fold the sides over and press down the edges with a fork. Brush some water on the edges.
Place the baking sheets in the oven and bake for 10 minutes. Rotate the sheets and bake for 10 more minutes. Take out and serve!