Rinse the prawns and pat dry with kitchen paper. Peel and chop the onion and garlic. Halve the chillies lengthwise and deseed. Wash the tomatoes, scoop out the seeds and cut into small cubes.
Heat the oil in a pan over a medium-high heat. Lightly season the prawns with salt and sear with the chilli for 1 minute on each side.
Remove the prawns from the pan and set aside. Add the garlic, onion and tomatoes to the pan and fry for 2 minutes. Pour in the white wine or lemon juice.
Return the prawns to the pan. Cook for 2 minutes and season with salt and black pepper. Drain the capers. Wash and dry the rocket. Mix both into the pan and serve.