First, boil the sweet potato with some salt and turmeric powder. Mash it lightly. Heat oil in a pan. Temper mustard seeds and cumin seeds. Then add and fry urad dal till it turns golden brown. Mash and add ginger and garlic to it along with minced green chillies.
Next add the onions and saute till they turn translucent. Now add the tomatoes, curry leaves, salt and asafoetida. Saute till the tomatoes are cooked. Next add the bell peppers and green peas. Saute till the veggies are cooked. Add the mashed sweet potato and give it a good mix.
It's time to add the seasonings like turmeric powder and coriander cumin powder. Lastly garnish it with cilantro. Mix it all together and take this mixture off the flame. Heat a skillet/tava, Pour a dollop of dosa batter and spread it like a pancake. You may add oil around the edges if needed. Add a spoonful of the sweet potato mixture in the center and let it cook for a minute.
Fold both ends and close the dosa. Take it off the skillet. Serve with any chutney/sambhar.