Remove the contents into a vessel and add cooking soda and mix well. The batter should be thick enough to pour in a small saucepan.
Pour a spoon of oil in a hot saucepan and pour 2 ladle full of the dosa batter into it. Hold the saucepan and turn it around so that the entire saucepan is coated with the dosa batter. Pour a tsp full of oil around the edges of the dosa and keep it covered. Take care to keep the stove on a medium flame. After a few minutes, turn the dosa. Keep it covered again. Remove when both sides are done.
Grind all the ingredients in a mixer to a fine paste.
Heat the remaining oil in a saucepan and add the mustard seeds to it. When the seeds splutter, add the ground onion paste into it and fry well till the oil leaves the sides of the pan.
:-This chutney can be preserved for a week, if fried well.