Heat the oil in a heavy-based frying pan. Add the onions, garlic, chilli and lemon rind and fry over a medium heat, stirring occasionally, for 1–2 minutes. Add the tomatoes and stock and bring to the boil. Lower the heat and simmer for 5 minutes.
Add the prawns, season to taste with salt and pepper and cook, turning occasionally, for about 4 minutes, until the prawns are pink. Garnish with the mixed herbs and serve immediately.