Heat oil in a large pan over medium heat. Add onion, garlic, ginger, and green chilies, frying until soft and fragrant.
Add red pepper to the pan and fry for 2 minutes until slightly softened.
Add all dried spices (coriander, cumin, chilli powder, turmeric, and garam masala) and fry until fragrant, about 1 minute.
Stir in chopped tomatoes, brown sugar, tamarind, and water. Bring to a simmer and cook for 10 minutes until the sauce has thickened.
Add the king prawns to the sauce and cook for 5 minutes until they are pink and cooked through.
Remove from heat and finish with fresh lemon juice and chopped coriander before serving.