Boil potatoes and green peas, peel, and lightly mash.
Heat 1 tbsp oil in a pan, crackle mustard seeds and curry leaves; sauté sliced onion until soft.
Add turmeric powder, salt, and mashed potato mixture; cook for 4 minutes into a dry filling.
Pour a ladle of dosa batter onto a hot griddle, spreading thinly in circles.
Drizzle oil around edges, place filling in center, fold when crispy, and serve hot.