Heat 1 tablespoon of the oil in a frying pan. Add the onion and cook for 5 minutes until softened, then add the garlic, ginger and cumin and cook for a further 1 minute. Stir in the prawns and cook for 3 minutes, then add the tomatoes and cook until the prawns are pink and cooked through.
Mix together the eggs, garam masala and chilli and season well. Heat 1/2 tablespoon of the remaining oil in a small, nonstick frying pan. Add one-quarter of the egg mixture and swirl around the pan. Stir once, then leave to cook for 1–2 minutes until starting to set. Spoon over one-quarter of the tomato prawn mixture and top with a little coriander. Place on a warmed chapatti and fold in half. Repeat with the remaining ingredients.