Heat sesame oil in a pan and add in the fennel seeds, cardamom and curry leaves. Let it sizzle. Add in the finely chopped onions and fry for 5-6 minutes until the onions are brown. Add in the tomatoes, ginger garlic paste, spice powders and the salt. Saute for 5-6 minutes on medium flame until the tomatoes are cooked and the oil starts separating from the mixture. The mixture should be dry by now. Add in the cleaned and de-veined prawns to the mixture. Prawns cook fast. Saute for 4-5 minutes until the prawns are fully cooked. Switch off the flame and garnish the prawn masala with coriander leaves.