Mix all the batter ingredients in a large bowl. Ensure there are no lumps.
Heat oil in a kadai and add in all the tempering ingredients in the same order as listed above starting with mustard seeds and ending with ginger.
Saute well till the onions turn transparent. I usually do this in the dosa tawa itself.
Add the tempering to the batter and mix well. Adjust the seasoning. The batter should be slightly watery.
Heat a dosa pan and pour the batter in a circular manner starting from outwards towards center. Fill any holes with small drops of batter.
Drizzle a tsp of oil around the dosa and cook one side on a medium flame till the edges start to lift slightly. Flip the dosa and cook for a minute. Remove the dosa and continue making dosas till the batter is used up.