To make the dosa batter, soak rice, urad dal, and fenugreek seeds in water separately for at least 6 hours or overnight. Then, grind them to a smooth batter using a wet grinder or mixer grinder. Add salt to taste and water as needed to adjust the consistency. Let the batter ferment for at least 8-10 hours or overnight.
To make the filling, heat oil in a pan and add mustard seeds and cumin seeds. When they start to crackle, add chopped onions, green chilies, and ginger. Saute for a few minutes until the onions turn translucent.
Add grated coconut, mashed potatoes, turmeric powder, and salt to taste. Mix well and cook for 2-3 minutes. Turn off the heat and let the filling cool down.
Heat a dosa tawa or griddle and spread a ladleful of dosa batter to make a thin dosa. Drizzle some oil around the edges and let it cook for a minute.
Place a tablespoon of the filling in the center of the dosa and fold the edges over the filling to create a triangular shape. Cook the dosa on low-medium heat until it is crispy and golden brown on both sides.
Serve hot with coconut chutney and sambar. Enjoy your delicious Dilkhush Dosa!