Heat oil in a pan and add mustard seeds. Once they start splattering add add split black gram and chopped onions, green chilies and curry leaves. and sauté until the onion turns translucent.
Now add the mashed potatoes, green peas and turmeric powder and add little water for the peas to cook. (You can add more water if you prefer masala to be in semi-solid consistency). Cook it for 7 to 10 minutes till the peas is completely cooked and garnish with coriander leaves and keep it aside.
Heat a skillet and coat with oil. Add dosai batter as needed and spread as thin as possible to a round shape. Once the dosai is cooked golden brown at the bottom and top is cooked spread masala on the dosai and roll it up and serve,
you can sprinkle ghee around the dosai or on dosai if you prefer.