Clean prawn and add salt and turmeric. Allow to marinate for 15 mins.
Grind the cumin seeds, 2 cardamom, 1 inch cinnamon and red chillis into a powder. Add the small onion, half of the garlic flakes and 1 inch ginger to the same grinder jar. Grind till the paste is smooth.Keep aside.
Finely chop the tomatoes and keep aside. Cut the potato into large cubes.
Coarsely grind the remaining onion and keep aside. Crush together the remaining garlic pod and ginger using a mortar and pestle. Take care not to make a very fine paste.
Heat 1 tbs oil in a pan. Add the marinated prawns and fry till golden. Remove and keep aside.
Add another tbs of oil into the wok. Add the potato pieces. Fry for 2-3 minutes. Remove and keep aside.
Add the ground masala along with turmeric and chilli powder. Fry for 4-5 minutes till the raw smell goes off.
Add tomato and fry for another 3-4 minutes or till it turns mushy.
Add 3 cups of boiling water to the wok. Bring to a boil on high flame.
Add fried potatoes and prawns to the wok. Cover with a lid and allow to boil for 7-8 minutes or till potatoes are cooked.
Powder the remaining cinnamon and cardamom and add to the curry. After 1-2 minutes, switch off the flame.
Serve hot with white rice.