Put the oil in a large non-stick frying pan and set over a medium-high heat. When hot, add the panch phoron and green chilli and fry for 30 seconds, then add the chopped tomatoes and fry for 5 minutes until the oil starts to separate from the tomatoes slightly.
Add the prawns, ground turmeric, chilli powder, sugar and mustard paste and fry for 3 minutes. Then add the frozen peas, 250 ml water and salt and stir until everything is mixed.
Cover and simmer for 6–8 minutes until the prawns are cooked through.