Chop 2 onions finely. Heat oil and add mustard, urad dal, green chillies(cut into pieces) and ginger. Add 1 1/2 cup water to the wheat flour and make it thin to a spreading consistency. Add onions(fresh onions can be added or if desired you can fry them with green chillies), fried green chillies and salt. Heat a tawa and pour the batter. Spread to a medium size round dosa. Pour oil around its circumference. Cook both the sides as you would in case of dosa.