Prawn karahi

Name: Prawn karahi
Prep time: 10 min
Cook time: 20 min
Total time: 30 min
Ingredients: 450 g Prawns, fresh raw; 1 Chilli, green; 2 tbsp Coriander, leaves; 3 1/2 1cm piece Ginger; 1/2 Lemon; 4 Tomatoes, large ripe; 2 tbsp Concentrated tomato puree; 1/2 tsp Kashmiri chilli powder or other chilli powder or paprika; 1/4 tsp Turmeric, ground; 1 tsp Cumin seeds; 1/2 tsp Nigella seeds; 2 tbsp Ghee; ½ tsp ajwain seeds (optional)