Clean and devein 250 grams of prawns, leaving the tails intact. Pat them dry with a paper towel and set aside.
In a mixing bowl, combine 2 tablespoons of cornflour, 2 tablespoons of rice flour, 1 teaspoon of red chili powder, 1/2 teaspoon of turmeric powder, 1/2 teaspoon of garam masala, 1/2 teaspoon of ginger-garlic paste, and salt to taste. Mix well.
Add the cleaned prawns to the spice mixture and coat them evenly. Allow the prawns to marinate for 15-20 minutes.
Heat oil in a kadai or deep frying pan over medium heat. Once the oil is hot, carefully add the marinated prawns and fry them until they turn golden brown and crispy. This should take about 3-4 minutes per side. Remove the fried prawns from the oil and place them on a paper towel to drain excess oil.
In a separate pan, heat 1 tablespoon of oil over medium heat. Add 1/2 teaspoon of mustard seeds and let them splutter. Then, add 1 sprig of curry leaves and 2-3 chopped green chilies. Sauté for a minute.
Add the fried prawns to the pan and toss them with the tempered spices. Cook for another 2-3 minutes, ensuring the prawns are well coated with the spices.
Remove from heat and serve hot with a side of lemon wedges and sliced onions.