In a mixer jar, add 1 cup rice flour, 1 cup besan (gram flour), ½ cup rava (semolina), 1 cup chopped raw potato pieces, ½ cup curd, salt to taste, ½ tsp sugar, and 1½ cups water. Grind until you get a smooth batter.
The batter should have a medium-thick, ribbon-like pouring consistency. Transfer it to a bowl, cover, and let it rest for 10 minutes.
Add ½ tsp baking soda and 1 tsp lemon juice. Mix well.
Heat a tawa, sprinkle a little water on it, and wipe it clean with a cloth to slightly cool the surface.
Pour 2 small ladles of batter onto the tawa and spread it evenly into a thin dosa.
Drizzle a little oil around the edges and cook over medium-low heat until the dosa turns golden and crispy.
In a small bowl, mix 1 tbsp red chilli powder, 1 tsp black salt, and 1 tsp chaat masala. Sprinkle this masala evenly over the dosa, then top with chopped onions and chopped capsicum. Cook for another 2 minutes until the vegetables are slightly roasted and the dosa becomes extra crispy.
Fold the dosa and serve it hot.