Pat the prawns dry, then toss with 1/2 tsp turmeric, 1/2 tsp Kashmiri chili powder, lemon juice, and a pinch of salt. Set aside for 10 minutes.
Heat 1 tbsp oil in a pan over medium-high heat. Sear the prawns for 1 minute per side until pink and slightly curled. Remove to a plate immediately.
Add remaining oil to the same pan. Splutter cumin seeds and curry leaves until they crackle and release their aroma.
Add onions and cook for 8-10 minutes until deep golden brown. Stir in ginger-garlic paste and green chilies for 2 minutes.
Reduce heat to low. Stir in coriander powder, remaining turmeric, and Kashmiri chili powder for 30 seconds.
Add pureed tomatoes and salt. Sauté until the oil separates from the edges. Pour in 1/2 cup of warm water and simmer for 5 minutes.
Return prawns and juices to the gravy. Simmer covered for 2-3 minutes. Finish with garam masala and fresh coriander.