Heat a small saucepan. Add 2 tsp oil to it. When the oil turns hot, add the mustard seeds and wait until it splutters. Then add the curry leaves, minced ginger, minced green chilli and chopped onion. Fry until the onion turns translucent – 1-2 minutes. Switch off and pour this mixture into the bowl containing the leftover dosa batter. Mix well into the batter.