Grind all the ingredients for the red chutney to a paste. Heat and season a dosa tawa. Pour a small ladlefull of dosa batter onto the tawa and spread evenly with the back of the ladle to make a medium-sized dosa.
Drizzle half a teaspoon of oil around the dosa and cook over low heat for half a minute. Spread a little red chutney all over the dosa. Place some potato bhaji, onions, capsicum and tomatoes in the centre of the dosa.
Sprinkle the red chilli powder and chaat masala and stir the potato mixture lightly. Add a tablespoon of butter, stir and mash lightly with a potato masher, taking care not to damage the dosa.
Spread the mixture all over the dosa. Continue to cook the dosa over low heat till the underside turns golden and crisp. Gently fold over one side and transfer the dosa to a serving plate. Cut into pieces and serve hot.