To prepare filling: Heat oil in a nonstick skillet over medium-high heat. add onions and cook, stirring, until lightly brown, about 3 minutes. Remove from heat and set aside.
Using a potato masher, mash the potatoes until smooth, blending in yogurt and the reserved onions. Add feta, parsley, scallions, and salt and pepper. set aside.
To make dough: Set oven rack in lowest position. If you have quarry tiles or a baking stone, place on the rack. Preheat the oven to 450 Degrees F. Lightly oil 2 baking sheets or coat them with nonstick cooking spray.
In a large bowl, combine 1 cup of the flour with the baking soda and salt. stir in yogurt. gradually add just enough of the remaining flour until the dough has lost its stickiness and can be worked with your hands. Turn the dough out onto a lightly flour work surface and knead until soft and slightly elastic, 3 to 4 minutes. divide the dough into 8 equal pieces.
To assemble and bake breads: Working with one piece at a time, and keeping the remaining dough covered, flatten the dough with the palm of your hand, then either stretch or roll out the dough to a circle 6 to 8 inches in diameter.
Spoon 2 Tb of the filling on the the center of the dough. Gently stretch the edges of the dough over the filling until you have a dough-covered mound. Pinch pleats closed and then, with the palm of your hand, gently press down on the top of the mound to flatten it. Turn the bread over and press again on the other side. this will push the filling to the edges of the bread. It should now be not more than 1/2 inch thick and 7 to 8 inches in diameter.
Transfer the filled circles, pleated side down, to a prepared baking sheet. Once the first baking sheet is full bake the breads on the lowest rack (or with the sheet directly on the quarry tiles or baking stone)) for 6 to 8 minutes or until the breads are lightly browned on the bottom. Assemble the remaining breads while the first batch bakes. Remove the breads from the oven and transfer to a basket lined with a towel. Bake the remaining breads in the same fashion. Serve hot.