Heat oil and add mustard. Let it pop up
Add curry leaves and onion. saute till onion turns translucent.
Add ginger and salt, red chilly powder,turmeric powder, and mix well.
When oil seperates add mashed potato and cover it for some time.
Meanwhile heat a non stick pan .
Apply butter to the sandwich bread on both sides and toast it on tawa.
Apply tomato ketchup on the 4 bread slices.
Grate some cheese and spread some potato mixture on it.
Cover it with other bread slice and toast it or grill it until cheese melts.
Serve hot with ketchup or mayo.
