Fry the dosa in a hot, oiled frying pan or "tava" (traditional Indian frying pan). Put a scoop of batter in the pan and spread it into a thin round layer. Fry over medium heat until the bottom is golden brown and crispy and the batter is cooked. You can add a little oil or ghee around the edges to give the dosa a nice, crispy surface. When the dosa is ready, add the filling to the centre and roll up.
Next morning, grind the ingredients in a food processor until smooth.
Add more water if needed. The batter should be fairly runny.
Prepare the filling.
Sauté the onion slices for one minute with the spices in a generous amount of oil.
Add the diced potatoes and saute potatoes until cooked. Then add the yogurt and stir.
Making Dosa
Take large flat frying pan. Heat the pan to medium heat.
Put some oil in the pan. Pour one sixth of the batter into the pan. Swirl the pan so that the batter spreads smoothly. Add oil to the surface and smooth the surface very thinly with a baking spatula. Don't worry - the dosa will come off the pan when it's done.
When the dosa starts to curdle on top, and has become crispy on bottom, add the filling to the centre of the dosa. Using the spatula roll the masala dosa into a tube.