Rinse urad dal thrice and soak them in water for 5 to 6 hours.
Just before making the fritters, heat a wide pan with 1 tablespoon oil. When it is hot enough, add cumin seeds, mustard and red chilli. When the seeds splutter, add curry leaves, ginger and green chilies. Soon the leaves will become crisp. Turn off the stove. Add turmeric and add hing. Cool this completely.
Once it cools down, add yogurt to this along with ¼ teaspoon salt. Whisk it well until smooth. If it is too thick, then add little water to bring it to consistency. It should be neither too thick nor too runny. Taste test and add more salt if required. Set this aside.
Drain the water from dal and add it to the grinder/blender along with salt and 3 tablespoons water.
Blend it for a few minutes. Then scrape the sides and sprinkle more water only if needed and blend to a smooth and fluffy thick batter.
Ensure the blender does not become hot while you blend. Keep sprinkling water as needed and blend.
Transfer the batter to a bowl and beat it well in clock-wise direction for a minute to make it light and fluffy.
The consistency of the batter has to be thick, fluffy and light. ( If it is runny then add 2 to 3 tablespoons rice flour and mix. Make punugulu instead of garelu)
How to make perugu vada
In a large wide serving bowl, whisk together curd and salt. Then add water to make buttermilk and set aside.
Heat oil in a pan for deep frying on a medium heat. Check if it is hot by gently dropping a small portion of batter to the oil. It has to rise without browning a lot.
Then take small portions of the batter and shape to balls. Place the ball on a greased parchment paper. Flatten slightly and make a hole in the center. Grease your finger and transfer the vada to your fingers. Gently slide to the hot oil by shaking off your fingers. (You can also use a tablespoon to drop the batter & make balls.)
Let them fry and firm up for a few minutes, then stir and fry them evenly until crisp and golden.
Remove and add them directly to the bowl of buttermilk. Immerse them fully by pushing down with your fingers.
Once they soak up the buttermilk and soften in about 2 to 3 mins. Remove them one by one and gently squeeze to remove excess buttermilk.
Drop them in the seasoned perugu/ yogurt. Gently stir the yogurt to coat them. They will be delicate so handle them carefully.
Cover and rest for a couple of hours in the refrigerator until they become soft. Avoid leaving this in hot place as the yogurt will turn sour. You can keep it in the refrigerator.