Boil the potato drain the water. Cool the potato add turmeric powder and mash it well.
In a pan heat oil add mustard seed, black gram, curry leaves and wait until it start to splatter. Now add the onion and cook until golden brown. Now add green chili, garlic and sauté for 2 to 3 minutes.
Now add 1 cup of water, mashed potato and salt to taste. Simmer and cook until right masala consistency is reached.