Heat one tablespoon of oil, add chopped garlic and chopped mushrooms and saute. Add mixed boiled vegetables and saute for a few minutes. Add turmeric powder, cumin powder and mix.
Add half the finely chopped green chillies, mint and coriander leaves and salt to taste. Mix and saute for a while. Add grated boiled potato and chaat masala and mix well.
Take off the heat, transfer to a bowl and let it cool. Shape into one and a half inch long finger like rolls. Make a thick batter with cornflour and enough water.
Heat oil in a kadai. Dip the vegetable rolls in the flour batter and deep fry till golden. Or grill in a tandoor. Mix together yogurt, remaining finely chopped coriander and mint leaves and remaining finely chopped green chillies.
Place two vegetable rolls in a roomali roti, pour some yogurt mixture over. Sprinkle sambal (chopped tomatoes, chopped cucumber and chopped onions), chaat masala and black salt. Serve immediately.