Heat oil in a large frying pan over medium-high heat. Add onions. Sauté, stirring occasionally, for 2-3 minutes or until they are soft and translucent.
Add ginger, garlic and green chilies (if using). Sauté, stirring, for 1 minute or until fragrant.
Add ground turmeric, ground cumin, ground coriander, garam masala and red chili powder. Sauté, stirring, for a minute or until aromatic.
Add tomato paste. Sauté, stirring, for few seconds.
Add prawns and 1 cup water. Season with salt and pepper.
Bring to a boil. Reduce heat, and simmer, covered, for 5-10 minutes depending on the size of the prawns or until they are cooked through.
Add coconut milk. Simmer for 1 minute.
Garnish with coriander (cilantro) leaves.
Serve with rice or naan.