To make sauce put tomato puree in bowl. Add salt, sugar, garam masala, ground roasted cumin seeds, chili powder, coriander leaves, green chili, lemon juice and 1 tablespoons water; mix well. Slowly ad coconut milk or cream, mix. Set aside. To stir-fry prawns put oil in wok and set over med-high heat. When hot put in mustard seeds. As soon as seed pop, a few seconds, add garlic and curry leaves. Sir until garlic tuns medium brown and put in prawns. Stir until prawns turn opaque most of the way though and put in sauce. Turn heat to medium and just heat sauce through until it begins to simmer. By then prawns should be cooked through. Turn of heat. Serve with rice.