1. Preheat oven to 425°.
2. Weigh or lightly spoon flours into dry measuring cups; level with a knife. Combine flours, sugar, baking powder, baking soda, and salt in a large bowl; cut in butter with a pastry blender or 2 knives until mixture resembles coarse meal. Stir in onions. Combine yogurt and egg white, stirring with a whisk. Add to flour mixture, stirring just until moist (dough will be sticky).
3. Turn dough out onto a lightly floured surface; knead lightly 4 times with floured hands. Pat dough into an 8-inch circle on a baking sheet lined with parchment paper. Cut dough into 12 wedges, cutting into but not through dough. Bake at 425° for 15 minutes or until golden. Serve warm.