Soak Rajma for 3 hour. Pressure cook it with the potato in 3 cup water for 2 to 3 whistle till they are just perfectly done. Or you can boil Rajma till they are done.
Once they are done just smash them coarsely with the back side of spatula. Mix mashed boiled potato ,gram flour to it.
Add lemon juice, black salt ,amchur powder (dry mango powder),chopped green chili and roasted cumin powder.Adjust salt.Leave it for 10 minute.
Now make 4 to 5 patty out of it .Heat a flat skillet and cook the patty at medium flame with a little oil.
Cut the onion into round rings .Once you are done with the cooking of burger patties ,put them on hot skillet and switch off the flame.
Make your burger placing the burger patty ,onion and kasundi sauce.Serve hot.