In a large bowl, combine flours, baking soda, salt, sugar and whisk well. Add sourdough discard, buttermilk and ¾ cup of water. Mix and knead to make a smooth and soft dough. Add more water, if needed. Knead well for 5~10 minutes. Cover with kitchen towel and set aside for 30 minutes.
Divide the dough into 8 equal pieces. Roll each piece into a smooth ball and place them on a lightly greased plate. Cover and let rest for 30 more minutes. Dough does not rise as much as a yeasted dough would, but will definitely puff up a little bit.
While the dough rests, prepare the filling - combine chopped onions, green chilies, cilantro and salt in a small bowl. Set aside until ready to use.
Preheat to 450°F. Place a baking stone or overturned baking sheet in the oven.
Take a piece of dough and roll it out on a lightly floured surface into a 5" round disc that is ¼" thick. Keep aside. Roll another piece of dough in the same manner. Spread ¼ of the onion filling on the rolled out dough and place the first dough on top of it. Lightly press to seal the edges.
Gently place the kulcha on the baking stone and bake for 4~5 minutes or until the dough is cooked through and some blisters start to appear. Repeat with the remaining 6 dough balls to make a total of 4 kulchas.Serve warm with dal or raita.