In a medium size mixing bowl, combine all purpose flour, rice flour, sooji, and salt. Add yogurt and 1 cup of water. Mix well making sure that there are no lumps. The mixture will be thick at this point. Cover and set aside for at least 30 minutes.
When ready to make dosa, stir in the remaining 1 cup of water and whisk well. Add cumin seeds, chopped onions, green chilies and cilantro into the batter. The batter should have a thin buttermilk consistency. Add more water if needed.
Heat a tawa or griddle on medium-high flame until very hot. Pour batter using a ladle from about 3"-4" above the tawa in a circular motion from outside to the center. Don't spread the batter.
Pour 1~2 teaspoons oil on the dosa and cook until golden brown and crispy, about 3 minutes. When the bottom is golden brown and crispy, gently flip and cook on the other side for about 2 minutes. Remove from the griddle and serve hot. Repeat with the remaining batter.