Mix the onion slices with vinegar, red chilli powder and salt. Knead the atta with salt, warm water and a few drops of oil into a smooth dough and make chappathis. Whisk the eggs, pepper powder and salt. Heat oil in a pan. Pour some egg mixture into it (like you would do for making an omelette). Place a chappathi in the centre. Turn this to the other side carefully, cook for a few seconds and place it on a plate. Spread the onion mix and chicken pieces evenly. Drizzle with tomato ketchup. Roll and wrap in Aluminum foil. Serve hot.