Finely chop the onion, carrot, green capsicum, curry leaves, and fresh coriander.

Combine the semolina, plain yogurt, salt, and water in a mixing bowl until a thick, lump-free batter forms.

Rest the batter for 10 minutes so the semolina absorbs the liquid.

Heat 1 tablespoon vegetable oil in a frying pan, then add 1 teaspoon mustard seeds and 1 teaspoon cumin seeds.

Add 1/2 teaspoon asafoetida and 8 curry leaves, then let the spices crackle for 30 seconds.

Add the chopped onion and sauté for 2 minutes until softened.

Add the carrot and green capsicum, then sauté for 3 minutes until slightly tender.

Stir in the mashed potato, 1 teaspoon ground turmeric, 1 teaspoon red chilly powder, and salt.

Cook the potato filling for 2 minutes, then mix in 1/4 cup fresh coriander.

Transfer the filling to a bowl and let it cool until safe to handle.

Grease the idli mold lightly with oil, then heat water in the idli steamer.

Mix the fruit salt into the rested batter immediately before filling the molds.

Spoon batter into each idli mold until half full, then place 1 tablespoon potato filling in the center.

Cover the filling with more batter, filling each mold about three-quarters full.

Steam the stuffed idlis for 12 minutes over medium heat until firm and springy.

Rest the steamed idlis for 2 minutes, then unmold them carefully with a spoon.

Heat 1 tablespoon vegetable oil with 1 teaspoon mustard seeds and 7 curry leaves for the tempering.

Stir in 1 1/2 teaspoons red chilly powder and the remaining fresh coriander, then drizzle the tempering over the idlis.