Slice the onions. Peel the boiled potatoes and mash them roughly with your hands. Do not make a paste.
Heat oil in a pan and add mustard seeds, chana daal, urad daal. Let the mustard seeds pop and the dals change it's color. Add asafoetida, curry leaves, ginger, and red chilies. Add turmeric powder and mix all.
Add sliced onion and cook until it gets translucent. Add mashed potatoes. Add salt to taste. Mix everything and cook for another 5 minutes. Garnish with chopped cilantro.
Heat tava or griddle. Get a laddle full of dosa batter and pour onto tava. Spread the batter into circular motion with the help of a laddle. Drizzle some oil along the side and top of the dosa. Let it cook until the edges turn brown. Add ⅓ cup of potato masala in the middle and fold the dosa. Serve immediately with coconut chutney.