Mix spices of your choice into the mashed potato. I prefer ½ onion, a little salt and very little red chilli powder. Spread the potato mixture on one side of the bread slices.
Heat a tava or a non-stick griddle with a spoon of oil. Place the bread slice on it, with the potato mixture face down on the oil.
Add a little salt to the hung curd and spread it over the bread slice on the pan. Make sure all the sides are covered with curd.
When the bread slice is completely covered with curd, lid the pan, ensuring that the lid does not touch the curd. Keep the flame low.
Take the lid off after a minute. By this time, the curd will have settled down uniformly all over the bread slice, indicating that the idli is ready. Use a broad spatula to lift it from the pan to make sure it doesn't break. Serve with tomato sauce.